Dissertations / Theses on the topic 'Carbon cycling'

We present two novel stable isotope methods for measuring lake metabolism and compare the results to traditional techniques. The delta 18O method measures planktonic gross primary production (GPP) from dissolved oxygen concentrations, isotopes and respiration (R) and the delta 13C method measures "whole-lake" GPP and R from dissolved oxygen and carbon concentrations and isotopes. All three methods showed GPP was greater than R over the ice-free season and estimates of GPP were not significantly different. There was also no significant difference in R as measured by bottle incubations and the delta13C method. However, the delta 13C method does not account for inputs of external carbon which will result in underestimation of R and overestimation of GPP. In systems with significant allochthonous carbon inputs, the delta13C method cannot be accurate unless these inputs are accounted for. The delta18O method was used to measure metabolic parameters of twenty-one northern temperate lakes and showed GPP dominated over R during the ice-free season. GPP and R were most strongly correlated with lake temperature, which in turn is a function of the amount of solar radiation received by the lake. Our results imply that it is this solar radiation that drives planktonic gross primary productivity, which in turn drives the majority of planktonic respiration. Variation in dissolved organic carbon only explained 8% of the variation in planktonic R, while variation in planktonic GPP explained approximately 80% of the variation in planktonic R. Despite general autotrophy in the lakes, they were generally oversaturated in CO2 during the ice-free season, on average 252+/-25%. However, we found little evidence to conclude that this was the result of an excess of in situ respiration over production. The magnitude of the annual excess of R over GPP was not sufficient to account for the flux to the atmosphere. Moreover, carbon evasion was not a function of respiratory flux, nor did the isotopic signature of dissolved CO2 in the lakes present evidence of respiration. Groundwater inputs of carbon dioxide represent a plausible source for carbon dioxide oversaturation in some but not all of the lakes sampled.

The relationship between temperature and soil respiration has been well explored although uncertainties remain. This thesis examined the relationship between temperature and rates of heterotrophic respiration in soils from three adjacent sub-alpine Australian vegetation types; woodland, shrubland and grassland. Temperature sensitivity of soil (Q10) has recently been a hotly debate topic, one side concluding that decomposition of recalcitrant, less labile components of soil organic matter are insensitive to temperature. Whilst others argue that there is no difference in the temperature sensitivities of labile and recalcitrant carbon pools. Robust modeling of rates of soil respiration requires characterization of the temperature response of both labile and recalcitrant pools. Laboratory incubation provides a means of characterizing the temperature response of rates of respiration whilst reducing the confounding effects encountered in the field, such as seasonal fluctuations in temperature, moisture and substrate supply. I used a novel system that allowed laboratory measurement of gas exchange in soils over a range of temperatures under controlled conditions. Measurements included CO2 efflux and O2 uptake over a range of temperatures from 5 to 40oC, characterization of temperature response and sensitivity, and respiratory quotients. Rates of heterotrophic respiration fitted both exponential and Arrhenius functions and temperature sensitivity varied and depended on the model used, vegetation type and depth in the soil profile. Long-term incubation indicated both labile and resistant pools of carbon had similar temperature sensitivities. Respiratory quotients provided a strongly predictive measure of the potential rate of decomposition of soil C, independent of the temperature response of respiration, providing a tool that may be used alongside derived parameters to help understand shifts in microbial use of C substrates. Vegetation type influenced soil chemical properties and rates of heterotrophic respiration. Rates of respiration correlated well with concentrations of carbon and nitrogen as has been previously observed, unlike previous studies however a positive correlation was observed between indices of plant available phosphorus and respiration. The soils examined were from three adjacent vegetation types formed on common geology, I concluded that vegetation type had a significant influence on soil, in contrast to the commonly held view by ecologists that soil type drives patterns in vegetation. Climatic effects such as longer, dryer hotter summer, reduced snow cover and increased incidence of extreme weather events such as frosts and bushfire are likely to drive patterns in vegetation in this region and therefore have a significant impact on carbon cycling in Sub-alpine Australian soils.

Glaciers and ice caps are recognised as an important component of the global carbon cycle. Carbon within glacial systems exists in organic and inorganic forms, across supraglacial, englacial and subglacial realms. It is often difficult to detach cryospheric carbon cycling from hydrology, with the transfer of carbon between glacial inventories relying upon meltwater flows. Classical glacial hydrology consists of distributed drainage delivering delayed flow meltwaters, throughout the accumulation season, superseded by quick flow, aerated channelized drainage during increased ablation. It is upon this template that most existing studies have addressed the dynamics of carbon within glaciated catchments. However, Icelandic glacial systems provide an opportunity to investigate the role of subglacial volcanism in driving carbon dynamics. Hydrochemical properties of Sόlheimajökull bulk meltwaters indicate untraditional redox conditions, with discharge of reduced, anoxic meltwaters in Summer, when expansion of subglacial drainage intersects the Katla geothermal zone. This unique hydrological regime generates profound effects upon the solute flux from the glacier, particularly with regard to the carbon budget. Dissolved inorganic carbon dynamics are dominated by weathering of basaltic bedrocks and accessory hydrothermal calcites, fuelled by subglacial geothermal proton supply. Widespread basal anoxia during summer facilitates methanogenesis resulting in large quantities of methane being discharged from beneath the glacier (flux range between 9,179 to 22,551 tonnes per year). Evidence suggests subglacial microbial acetoclastic methanogenesis is responsible with δ13C and δD CH4 values of ~60‰ and -320‰ respectively, supported by laboratory identification of methanogenesis in Sόlheimajökull subglacial sediments. The organic counterpart to the carbon cycle is invoked to serve as the energy source for microbial metabolism. Such direct measurements of subglacial methane have rarely been achieved at contemporary ice margins. This study therefore provides an exciting opportunity to identify methane sources and carbon cycling in areas subjected to subglacial volcanism and to consider these within the broader context of global carbon dynamics.

The Cretaceous (~145–65 Ma) is widely regarded as a greenhouse period with warm, equable climates and elevated atmospheric CO2 relative to the modern. However, the earliest Cretaceous (Berriasian–Barremian; 145–125 Ma) is commonly characterised as a relatively colder “coolhouse” interval, typified by lower global temperatures than the mid-Cretaceous. Unfortunately, the lack of absolute sea surface temperature (SST) estimates prior to the Barremian has hampered efforts to definitively reconstruct Early Cretacous climate. Here, the TEX86 palaeotemperature proxy, for which a detailed review is provided, has been used to generate a 13 myr record of SST estimates for the Early Cretaceous, based on sediments from assorted deep-sea drilling sites. A consistent offset in the TEX86 ratio between transported mudstones and pelagic carbonates in the low-latitude marine sediments (DSDP Sites 603 and 534) has been identified, which may be linked to post-burial diagenesis or a difference in organic matter type between lithologies. Mindful of these apparent lithological effects on TEX86, only the pelagic sediments were used to subsequently reconstruct Early Cretaceous SSTs. These TEX86 records demonstrate both elevated SSTs (>27 ºC) at low and mid-latitudes relative to the modern, and the apparent stability of these high temperatures over long timescales. This lack of SST variation in the low-latitudes during the Valanginian positive carbon-isotope event (CIE; ~135–138 Ma), casts doubt on the warming-weathering feedback model put forward to explain this major perturbation. Additionally, new paired bulk organic (δ13Corg) and bulk carbonate (δ13Ccarb) carbon-isotope records from North Atlantic DSDP sites, have been used to reconstruct relative changes in pCO2 across the CIE. These observed fluctuations in Δ13C imply changes in carbon-cycling and a possible drawdown in CO2, due to excess organic matter burial associated with the CIE.

Savannah ecosystems play a prominent role in the global carbon (C) cycle, yet fluxes are poorly quantified, and the key processes regulating vegetation dynamics are uncertain. Insight is particularly deficient in southern Africa’s miombo woodlands, a woody savannah that is home to over 100 million people. This biome is heavily disturbed, with widespread deforestation and degradation associated with agriculture, charcoal and timber extraction, and frequent fires from anthropogenic sources. In this thesis I combine plot inventory data with remote sensing and modelling techniques to improve our understanding of the miombo woodland C cycle. Using a network of forest inventory plots, I characterise floristic and functional diversity in a savannah-forest mosaic in southeastern Tanzania. Divergent vegetation structures are associated with variation in fire frequency, water supply, and soil chemo-physical properties. Corresponding differences are noted in fire resilience, water-use, and nutrient acquisition plant functional traits, suggesting that multiple interrelated environmental filters act to assemble heterogeneous tree communities. Re-inventory of forest plots was used to quantify key aspects of the woody C cycle. Tree growth rates are slow, calling for careful management of woodland resources, and significantly reduced where stems were damaged. Stem mortality is rare, though elevated in the smallest trees and where damage was recorded. Contemporary strategies to incentivise the conservation of miombo woodland ecosystems, such as the REDD+ programme of the United Nations, advocate payments for sustaining ecosystem services such as C sequestration. I report on a pilot REDD+ project aiming to reduce woodland degradation from frequent high intensity fires in southeastern Tanzania. Model simulations suggest that woody biomass is being gradually lost from the region, and that setting early season fires has the potential to reverse this trend. Realising substantial changes in C storage requires a demanding reduction to late fire frequency, and uncertainty in model predictions remains high. I quantify the C cycle of southern African woodlands by combining observational data with a diagnostic C cycle model under a model-data fusion framework. Model outputs show substantial variation in primary production, C allocation patterns, and foliar and canopy traits, which are associated with differences in woody cover, fire, and precipitation properties. C cycle dynamics correspond poorly to conventional land cover maps, indicating they may be unsuited to upscaling measurements and models of the terrestrial C cycle.

Pacific Northwest coastal wetland extent has been significantly reduced due to development. To understand the effects of land use change on carbon cycling in coastal wetlands, we compared soil carbon dynamics in restored, disturbed (by diking or draining), and reference wetlands in both freshwater and saline conditions in Coos Bay, Oregon. We quantified soil carbon pools, measured in situ fluxes of methane (CH4) and carbon dioxide (CO2), and estimated sediment deposition and carbon sequestration rates. We found that land use change influences carbon cycling and storage in coastal wetlands. The disturbed marshes have likely lost all their organic material after draining or diking, except for a shallow A horizon. The restored marsh in situ CH4 and CO2 fluxes were intermediate between the disturbed and reference marshes. Generally, restored marshes showed a partial return of carbon storage functions, or an indication that reference level functions may be achieved over time.

Peatlands are a globally important, terrestrial store of carbon and the UK is recognised as an internationally significant holder of peatlands. Of all the kinds of peatland found in the UK, blanket bogs are dominant, representing 87% of the UK’s peatland area. The UK’s peatlands, in contrast to many other areas of boreal/temperate peat, are relatively accessible and as such have been subject to land-management pressures for many thousands of years. These management pressures have led to the deterioration of many peatlands in the UK, with only 1% of England’s peatlands being considered ‘pristine’ in a Natural England report (Natural England, 2010). Climate change and increasing land-use pressures are predicted to affect all UK peatlands in coming years. As such, studies of the drivers of carbon cycling on UK peatlands are being undertaken in order to help in the construction of models to predict the dynamics of peatland carbon balance. These models will subsequently enable land-managers and policy makers to take informed decisions regarding peatland management and carbon storage. One such model of peatland carbon balance is the Durham Carbon Model, which uses a mass balance between fluxes of carbon in and out of a peatland in order to estimate its net carbon budget. While the Durham Carbon Model is able to deal with the effects of some aspects of land-management on peatland carbon balance, there remain a number of important drivers as yet unaccounted for in the model. As such, the remit of this thesis was to conduct in-situ, experiments in order to provide additional data on peatland carbon cycling with a view to incorporating these drivers into the model. Specifically, this research examines three areas as yet unaccounted for in the Durham Carbon Model: altitude, vegetation and diurnal processes. These factors are considered relative to CO2 flux and, in some cases, soil pore water dissolved organic carbon concentration. Additional experiments were also performed to determine whether empirical models of CO2 flux can be physically interpretable. Results obtained for this thesis suggest that the most important factor in predicting CO2 flux on blanket peat soils is vegetation type and vegetation mediated processes, i.e. photosynthetic controls on respiration. Moreover, the relationship between respiration and photosynthesis was found across a range of other factors and temporal scales. In addition to vegetation, altitude was found to significantly affect CO2 for some vegetation types. Therefore, both of these factors are to be incorporated into the Durham Carbon Model. Experiments suggested that empirical models of CO2 flux can be physically interpretable. The results of the diurnal experiment gave evidence to support the hypothesis that some component of the relationship between photosynthesis and respiration is temporally lagged, perhaps by 3 hours. However, the results were not unequivocal and thus further work is needed to fully examine some of the results presented herein.

Land application of biosolids has been demonstrated to improve nutrient availability (mainly N and P) and improve organic matter in soils, but the effects of biosolids on C sequestration and N cycling in the Mid-Atlantic region is not well understood. The objectives were: 1) to investigate soil C sequestration at sites with a long-term history of biosolids either in repeated application or single large application; 2) to characterize and compare soil C chemistry using advanced 13C nuclear magnetic resonance (NMR) and C (1s) near edge x-ray absorption fine structure (NEXAFS) spectroscopic techniques; and 3) to compare biosolids types and tillage practices on short-term N availability in the Coastal Plain soils. Biosolids led to C accumulation in the soil surface (< 15 cm) after long-time application in both Piedmont and Coastal Plain soils. The C saturation phenomenon occurred in Coastal Plain soils, thus additional soil C accumulation was not achieved by increasing C inputs from biosolids to the Coastal Plain. Soil organic C from profiles in the field sites was not different at depths below the plow layer (15-60 cm). The quantitative NMR analyses concluded that O-alkyl C was the dominant form in the particulate organic matter (POM), followed by aromatic C, alkyl C, COO/N-C=O, aromatic C-O, OCH3 / NCH and ketones and aldehydes. The aliphatic C and aromatic C were enriched but the O-alkyl C was decreased in the biosolids-amended soils. The changes indicated that the biosolids-derived soil C was more decomposed and, thus, more stable than the control. The NEXAFS spectra showed that O-alkyl C was the dominant form in the POM extracted from biosolids-amended soils, followed by aromatic C, alkyl C, carboxylic C and phenolic C groups. These results were similar to those from NMR analysis. The regression and correlation analyses of C functional groups in the POM between NEXAFS and NMR indicated that both techniques had good sensitivity for the characterization of C from biosolids-amended soils. To evaluate short-term biosolids N availability, a three-year field study to investigate the effects of lime-stabilized (LS) and anaerobically digested (AD) biosolids on N availability in a corn-soybean rotation under conventional tillage and no-tillage practices was set up in 2009-2011. Results showed that both LS and AD biosolids increased spring soil nitrate N, plant tissue N at silking, post-season corn stalk nitrate N, grain yield, and soil total N by the end of the growing season. The same factors used to calculate plant available N for incorporated biosolids can be used on biosolids applied to no-till systems in coarse-textured soils. All these results indicated that the application of biosolids affects the long-term quantification and qualification of soil organic C and also improve short-term N availability in the Mid-Atlantic region.
Ph. D.

The tropical forests on the island of Borneo are among of the richest in the world in terms of tree diversity, and their capacity to store a large reservoir of carbon. The Southeast Asian forests are fundamentally different from Neotropical and African forests, with their single-family dominance by dipterocarp trees, and with inherently greater stature and biomass. The carbon productivity and allocation in Asian tropical forests is still poorly quantified, and their responses to environmental drivers are still poorly understood. Almost all recent advances in tropical forest carbon cycling research have occurred in the Neotropics, with very few studies in Asia. The principal aim of this thesis is to quantify the carbon budget of a lowland dipterocarp forest in the Lambir Hills National Park, Miri, Sarawak, Malaysian Borneo. I examined and explored the productivity and carbon cycling processes and their responses to environmental factors across two major and contrasting soil types, in particular the clay and sandy loam soils. I recorded and analysed the Net Primary Productivity (NPP) and respiration for the above- and below-ground components, and observed the responses to seasonal variation and environmental drivers. Total soil respiration was relatively high and contributed a great deal to ecosystem respiration. Variation in soil respiration rates appeared closely related to soil moisture content. I found a strong diurnal cycle in soil respiration. On the basis of the first soil carbon dioxide (CO2) efflux partitioning study undertaken in a tropical forest, the diurnal cycle in total soil respiration appeared to be entirely driven by the diurnal cycle in litter respiration, and in turn litter is strongly controlled by moisture. There was little seasonal variation in allocation of net primary productivity (NPP), but there was evidence showing potential inter-annual variability for several components of NPP. Further, the allocation of NPP showed a strong seasonal shift between the forest plots on clay and sandy loam soils. Combining all the data measured and obtained in this D.Phil. thesis, the overall carbon budget assessed in this lowland dipterocarp forest showed a high level of agreement with other studies in Asia using micrometeorological techniques and the situation appears to be comparable to tropical forests in Amazonia. The key difference is that the aboveground NPP is higher and is the largest component contributing to the overall carbon budget, with relatively higher carbon use efficiency (CUE). The lowland dipterocarp forest in Lambir shows higher allocation in the above-ground NPP, and there were also differences in NPP and its allocation between sandy and clay-rich plots.

The presented thesis investigates the role of bryophytes in the deadwood and carbon (C) cycle of boreal black spruce forests in Labrador, Canada. All major forest C pools (live-tree, standing and downed deadwood, organic layer, mineral soil) were quantified for three old-growth, nine clearcut harvested, and three burned forest stands in order to characterize forest C dynamics of a high-latitude humid boreal forest ecosystem. Tree and aboveground deadwood C dynamics of Labrador black spruce forests were similar to those of drier or warmer boreal forests. However, due to bryophyte-driven processes such as woody debris (WD) burial and paludification, the studied forests contained high organic layer, mineral soil, and buried wood C stocks. The comprehensive field-measured data on C stocks was used to evaluate the CBM-CFS3, a Canadian national-scale C budget model, with respect to its applicability to Labrador black spruce and humid boreal forests elsewhere. After selected biomass estimation and deadwood decay parameters had been adjusted, the CBM-CFS3 represented measured live-tree and aboveground deadwood C dynamics well. The CBM-CFS3 was initially designed for well-drained upland forests and does not reflect processes associated with bryophytes and high forest floor moisture content, thus not capturing the large amounts of buried wood and mineral soil C observed in the studied forests. Suggestions are made for structural changes to the CBM-CFS3 and other forest ecosystem C models to more adequately represent the bryophyte-regulated accumulation of buried wood, organic layer, and mineral soil C. Accuracy of forest C models could be further improved by differentiating WD decomposition rates by disturbance history, because WD respiration reflects disturbance-induced changes in temperature and moisture regimes. In Labrador, WD respiration was limited by low WD moisture levels and high temperatures in burned stands, and by high WD moisture contents and low temperatures in old-growth stands. Following harvesting, residual vegetation prevents the desiccation of WD, resulting in significantly higher WD respiration compared to old-growth and burned stands. Moreover, the bryophyte layer recovers faster following harvest than following fire, which reduces WD desiccation due to moisture retention, water transfer, and moisture-induced cooling and results in higher WD decomposition rates. Bryophytes are thus a key driver of the deadwood and C cycle of humid boreal Labrador black spruce forests. The author recommends to classify these and similar boreal forests as a functional ecosystem group called “humid boreal forests”, preliminarily defined as “boreal forest ecosystems featuring a bryophyte-dominated ground vegetation layer associated with low soil temperatures, high moisture levels, low dead organic matter decomposition rates, and subsequently (in the absence of stand-replacing disturbances) an accumulation of buried wood embedded in a thick organic layer”. Bryophytes are also an integral component of many coniferous forests outside the boreal biome. Bryophyte-regulated processes such as WD burial or paludification are thus likely significant to the global C cycle. The potential climate change-induced release of large amounts of CO2 from buried wood and soil C pools necessitates an increased understanding of how bryophyte productivity and decomposition constraints will change with increasing temperature and varying moisture regimes. Ecosystems such as humid boreal forests with potentially high C losses to the atmosphere may thus be identified and counteractive forest management strategies can be developed and implemented
Cette thèse de doctorat s’intéresse à l’influence qu’exercent les mousses sur les cycles du bois mort et du carbone (C) dans des pessières noires boréales humides du Labrador, Canada. Toutes les réservoirs majeurs de C (arbres vivants, bois mort sur pied et effondré, l’horizon de matière organique, sol minéral) de trois pessières vierges, neuf coupes à blanc et de trois pessières brûlées ont été quantifiés pour caractériser le cycle du C des forêts humides boréales du nord. Les dynamismes de C des arbres vivants et du bois mort supraterrestre ressemblaient à ceux des forêts boréales plus sèches ou aux températures plus chaudes. À cause des processus régulés par les mousses (l’enterrement du bois mort ou la paludification), les forêts étudiées contenaient des stocks élevés de C au sein de l’horizon de matière organique, le sol minéral et le bois enterré. Les données ont aussi été utilisées pour évaluer le MBC-SFC3, un modèle national canadien du bilan du C, concernant son applicabilité aux pessières boréales humides de Labrador et d’ailleurs. Suite à l’ajustement de quelques paramètres, p.ex. des taux de décomposition, le MBC-SFC3 reproduisait bien le dynamisme mesuré des arbres vivants et du bois mort supraterrestre. Le MBC-SFC3 a initialement été développé pour les sites bien drainés et ne considère pas les processus associés avec les mousses ou l’humidité élevée du sol. Conséquemment, le MBC-SFC3 ne représentait pas les stocks élevés de C mesurés pour le bois enterré et pour le sol. Les modifications structurelles du MBC-SFC3 et d’autres modèles du C forestier sont nécessaires pour représenter adéquatement l’accumulation du C au sein de ces réservoirs. La précision des modèles du C forestier pourrait encore être améliorée par une différenciation des taux de décomposition selon le régime de perturbations, parce que la respiration du bois mort reflète les changements de la température et d’humidité associés avec une perturbation spécifique. Dans les pessières brûlés du Labrador, la respiration du bois mort était limitée par a faible humidité du bois et des températures élevées; dans les pessières vierges, par l’humidité élevée du bois et des températures basses. Dans les coupes à blanc, la végétation résiduelle empêchait le dessèchement du bois mort. Il s’y ensuivit que la respiration du bois mort y est nettement plus élevée en comparaison avec des pessières brûlés ou vierges. La décomposition du bois mort après coupe à blanc est aussi favorisée par la récupération plus rapide de la couche de mousses, diminuant conséquemment le dessèchement du bois mort par la conservation d’humidité, les transports vertical et horizontale d’eau et le refroidissement induit par l’humidité. Ainsi, les mousses sont les facteurs clés dans les cycles du bois mort et du C des pessières noires boréales au Labrador. L’auteur préconise la classification de ces pessières et des forêts semblables comme un groupe fonctionnel d’écosystèmes nommé : « pessières boréales humides » ; provisoirement définies comme « des écosystèmes forestiers avec une végétation terrestre dominée par les mousses et par conséquent associée avec des températures basses du sol, une humidité élevée, des taux de décomposition faibles et (en l’absence de perturbations) l’accumulation du bois enterré dans des couches organiques epaisses ». En outre, les mousses sont des éléments principaux des nombreuses forêts résineuses n’appartenant pas au biome boréal. Les processus régulés par les mousses tels l’enterrement du bois mort ou la paludification sont probablement importants pour le cycle global de C. La libération potentielle de grandes quantités de CO2 des réservoirs « bois enterré » et « sol » à la suite des changements climatiques exige une meilleure compréhension des transformations de la productivité des mousses et des limitations de la décomposition dues aux températures plus élevées et au taux d’humidités variables. Ainsi, les écosystèmes aux pertes potentielles de C élevées (p.ex. les pessières boréales humides) peuvent être identifiés et des mesures d’aménagement antagonistes peuvent être développées et implémentées. Traduction assistée par : Karl-Heinrich von Bothmer, Géry van der Kelen
Die vorliegende Arbeit untersucht die Einflüsse von Moosen auf den Totholz- und Kohlenstoff-(C)-Kreislauf in borealen Schwarzfichtenwäldern in Labrador, Kanada. Um den C-Kreislauf dieses humiden borealen Waldökosystems zu charakterisieren, wurden alle bedeutenden C-Speicher (lebende Bäume, stehendes und liegendes Totholz, organische Auflage, Mineralboden) von drei Primärwald-, neun Kahlschlags- und drei Brandflächen quantifiziert. Die C-Dynamiken der Bäume und des oberiridischen Totholzes der Untersuchungsflächen ähnelten denen von trockeneren und/oder wärmeren borealen Wäldern, während die organische Auflage, der Mineralboden und das begrabene Totholz bedingt durch von Moosen regulierte Prozesse wie Totholzeinlagerung und Paludifizierung besonders hohe C-Vorräte aufwiesen. Mit dem umfangreichen C-Datensatz wurde das CBM-CFS3, das nationale kanadische C-Modell, am Beispiel Labradors im Hinblick auf seine Anwendbarkeit in humiden borealen Wäldern evaluiert. Nach Anpassung ausgewählter Parameter, z.B. der Totholzabbauraten, wurden die gemessenen C-Dynamiken der Bäume und des oberiridischen Totholzes vom Modell abgebildet. Das CBM-CFS3 wurde ursprünglich für staunässefreie, terrestrische Waldstandorte entwickelt und berücksichtigt keine mit Moosen oder hoher Bodenfeuchte assoziierten Prozesse, so dass es die hohen C-Vorräte des begrabenen Totholzes und des Bodens nicht widerspiegelte. Eine adäquate Abbildung der Akkumulation von C in diesen Speichern erfordert strukturelle Änderungen des CBM-CFS3 und anderer Wald-C-Modelle. Die Genauigkeit von Wald-C-Modellen könnte darüber hinaus durch eine Differenzierung der Totholzabbauraten in Abhängigkeit vom Störungsregime verbessert werden, da störungsspezifische Veränderungen von Temperatur und Feuchte von der Totholzatmung widergespiegelt werden. Im Untersuchungsgebiet limitierten geringe Holzfeuchten und hohe Holztemperaturen die Totholzatmung auf Brandflächen. In Primärwäldern wirkten dagegen hohe Holzfeuchten und geringe Holztemperaturen hemmend. Auf Kahlschlägen verhinderte die verbleibende Vegetation die Austrockung des Totholzes, was zu signifikant erhöhten Atmungsraten im Vergleich zu Brand- und Primärwaldflächen führte. Zudem wird der Totholzabbau auf Kahlschlen durch eine schnellere Erholung der Moosdecke als auf Brandflächen gefördert, da Moose durch ihr hohes Wasserspeichervermögen, vertikalen und horizontalen Wassertransport und feuchte-induzierte Kühlung der Austrockung des Totholzes entgegenwirken. Moose sind somit ein Schlüsselfaktor im Totholz- und C-Kreislauf der humiden borealen Schwarzfichtenwälder Labradors. Die Autorin empfiehlt die Klassifikation dieser und ähnlicher borealer Wälder als eine funktionelle Ökosystemgruppe namens “humid boreal forests”; vorläufig definiert als “boreale Waldökosysteme mit durch Moose dominierter Bodenvegetation und damit assoziierten niedrigen Bodentemperaturen, hohen Bodenfeuchten, geringen Abbauraten und (in Abwesenheit großflächiger Störungen) der Akkumulation von begrabenem Totholz in mächtigen organischen Auflagen”. Auch außerhalb des borealen Bioms sind Moose ein wesentlicher Bestandteil vieler Nadelwälder. Durch Moose regulierte Prozesse wie Totholzeinlagerung und Paludifizierung sind daher wahrscheinlich relevant für den globalen C-Kreislauf. Die durch den Klimawandel bedingte potentielle Freisetzung von großen Mengen CO2 aus begrabenem Totholz und dem Boden macht ein besseres Verständnis der zu erwartenden Veränderungen von Mooswachstum und Abbauhemmnissen als Folge erhöhter Temperaturen und variabler Feuchteverhältnisse erforderlich. Somit können Ökosysteme mit potentiell hohen C-Verlusten, wie z.B. humide boreale Wälder, identifiziert und diesen entgegenwirkende Bewirtschaftungsmaßnahmen entwickelt und umgesetzt werden

Understanding methane (CH4) cycling dynamics is of paramount importance because CH4 has 45 times the sustained-flux global warming potential of carbon dioxide (CO2) and is currently the second most important anthropogenic greenhouse gas. Wetland ecosystems emit one-third of total global CH4 emissions, making them the single largest natural CH4 source and placing them among the most important terrestrial ecosystems in the global carbon (C) cycle. Wetlands in tropical and boreal regions are drivers of recent inter-annual variation in atmospheric CH4 concentrations because they play vital roles in the global CH4 cycle by storing vast amounts of C (~31% of total soil C in boreal peatlands) and generating a significant proportion of total global wetland CH4 emissions (47-89% in tropical wetlands). However, despite the recognized importance of these ecosystems, tropical wetlands have received limited study concerning CH4 flux and, although boreal wetlands have been more thoroughly studied, significant questions remain surrounding the biogeochemical controls over CH4 dynamics in these systems. My dissertation addresses these concerns using a combination of in situ field measurements and controlled laboratory incubations across field sites in equatorial Gabon, Africa and at an experimentally-manipulated (surface and deep warming and atmospheric CO2 enrichment) peatland in northern Minnesota. Specifically, my research provides novel information about the rates and abiotic and biotic controls over methanogenesis and methanotrophy in tropical African wetland and upland habitats (Chapter II). This chapter paired functional datasets with corresponding measurements of microbial community composition, using a holistic research approach that provided unique ecological insights into tropical ecosystem CH4 cycling. In northern Minnesota, I investigated the C source fueling anaerobic C mineralization across a variety of boreal peatlands, as well as if methanogenesis was limited by labile C availability at depth (Chapter III). Finally, my dissertation includes novel results on the response of boreal peatland CH4 and CO2 production, as well as anaerobic oxidation of CH4 (AOM), to deep peat heating (Chapter IV; does not include AOM) and whole-ecosystem warming with atmospheric CO2 enrichment (Chapter V), expanding our mechanistic understanding of how climate-driven variables affect peatland C mineralization. This dissertation includes previously published and unpublished coauthored material.

Biological activity in desert soils is driven by water availability. The nature of individual precipitation events is critical to understanding soil moisture availability. Rain falls as discrete events (pulses) that vary in size and sequencing, resulting in soil "wet-dry cycles". Soil organisms are responsive to wet-dry cycles with rapid changes in activity. How soil activity is driven by changes in water content associated with individual pulses is poorly understood. The effects of precipitation on soil processes likely depend on ecosystem structure, which influences the soil environment. The goal of this dissertation was to determine how soil carbon cycling responds to precipitation in the context of ecosystem structure (plant composition, geomorphology) and climate.I used differences in stable carbon isotopic composition of soil organisms and plants to understand how positioning in the soil profile influences biological responses to different sized pulses. I evaluated how soil texture and grass species composition affect soil process response to rainfall in different seasons. I manipulated rainfall sequence to understand the interaction between closely spaced rainfall events of different sizes on soil processes. I evaluated the role of plant functional types in influencing soil microclimate and litter deposition and the response of soil processes to seasonal rainfall.Chamber measurements of soil and plant CO2 flux were used to understand their response to rainfall. I found that surface organisms are more responsive to small rainfall events due to the relationship between pulse size and infiltration. While soil texture and season of rainfall are important, the best predictor of the response of soil respiration to rainfall was initial activity levels. Grass species was not important. Grass roots and soil microbes differ in response to sequences of precipitation. Grasses responded less to subsequent large events if they were already 'activated' by a recent rainfall event. The effect of plant functional type was size dependent with differences occurring only with large shrubs. This work suggests that large scale simulations of soil carbon cycling in deserts should carefully consider wet-dry transitions in the context of plant functional type and initial soil condition in order to predict the responses to global change.

As ever-increasing levels of carbon dioxide alter the chemistry of the Earth’s atmosphere, understanding the global carbon cycle becomes increasingly important. A particularly important component is the riverine carbon cycle, as rivers are the primary conduits for dissolved inorganic carbon from terrestrial watersheds to ocean basins. Stable carbon isotopes (13C/12C) were collected weekly and input into the mixing model IsoSource to delineate seasonal carbon sourcing along two nested basins in the upper Green River System, Kentucky. In the more siliciclastic upstream catchment, dissolved inorganic carbon (DIC) was primarily derived from soil respiration (34%). Groundwater dissolving carbonate bedrock and carbonate dissolution/precipitation reactions contributed 31% and 11%, respectively. The more carbonate-dominated downstream catchment also was influenced greatly by soil respiration (35%). Due to the more pronounced levels of carbonate bedrock, carbonate reactions contributed double that of the upstream catchment (20%), with groundwater contributing 22%. Seasonally, the upstream basin gathered most DIC from soil respiration from late spring to winter. Early spring precipitation and still limited photosynthesis caused the primary carbon sourcing to shift to groundwater. Downstream, the primary source throughout the entire study period was soil respiration. Collectively, this study provides insight into the carbon cycling process in a mid-latitude, karstic river using carbon isotope sourcing to aid in the quantification of global carbon flux in the critical zone.

Water management that alters riverine ecosystem processes has strongly influenced deltas and the people who depend on them, but a full accounting of the trade-offs is still emerging. Using palaeoecological data, we document a surprising biogeochemical consequence of water management in the Colorado River basin. Complete allocation and consumptive use of the river's flow has altered the downstream estuarine ecosystem, including the abundance and composition of the mollusc community, an important component in estuarine carbon cycling. In particular, population declines in the endemic Colorado delta clam, Mulinia coloradoensis, from 50-125 individuals m(-2) in the pre-dam era to three individualsm-2 today, have likely resulted in a reduction, on the order of 5900-15 000 tCyr(-1) (4.1-10.6 mol Cm-2 yr(-1)), in the net carbon emissions associated with molluscs. Although this reduction is large within the estuarine system, it is small in comparison with annual global carbon emissions. Nonetheless, this finding highlights the need for further research into the effects of dams, diversions and reservoirs on the biogeochemistry of deltas and estuaries worldwide, underscoring a present need for integrated water and carbon planning.

Despite the importance of tropical rivers to the global carbon cycle, the nature of carbon cycling within these watersheds has been dealt with by only a handful of studies. The current work attempts to address this lack of information, using stable isotope and concentration measurements to constrain sources and sinks of carbon in two Peninsular Malaysian watersheds. The basins are located on the central-western and northeastern coasts of the Malaysian Peninsula, and are drained by the Langat and Kelantan Rivers, respectively. Water samples were collected from three points along the two rivers twice a month, in addition to the sampling of groundwater in adjacent aquifers. Principal component analyses (PCA) on water chemistry parameters in the Langat and Kelantan Rivers show the dominance of geogenic and anthropogenic influences, grouped in 4 to 6 components that comprise over 50 % of the total dataset variances. The geogenic input is reflected by components showing strong loadings by Ca, Mg, Mn, Si, and Sr, while anthropogenic influences via pollution are indicated via strong loadings by NO3, SO4, K, Zn and Cl. The carbon isotope and concentration data appear unrelated to these groups, suggesting that the riverine carbon cycle in both locations is dominated by other factors. These may include alternative sources of organic pollution, or inputs from the local vegetation and soils. The mean riverine 13CDOC of -27.8 ± 2.9 ‰ and -26.6 ± 2.2 ‰ in the Langat and Kelantan Basins, respectively, are consistent with the dominance of C3-type vegetation in both watersheds. Riverine 13CDIC signatures approach C3-like values at high DIC concentrations, with measurements as low as -19 ‰ in the Kelantan Basin and -20 ‰ observed in the Langat Basin, consistent with a biological origin for riverine DIC. However, the average 13CDIC in river water is 13C-enriched by about 10 ‰ relative to the expected C3 source in both rivers, and this 13C- enrichment appears to be largest with smaller DIC concentrations. Because of the overpressures of CO2 in the rivers, entrainment of isotopically-heavy atmospheric CO2 is not a likely explanation for the observed 13C-enrichment. Theoretically, dissolution of carbonates could be an alternative source of 13C-enriched carbon, but this lithology is scarce, particularly in the Langat watershed. The increase in DIC downstream and generally high pCO2 values in most river sections argues against aquatic photosynthesis as a primary causative factor for the observed isotopic enrichment. This elimination process leaves the speciation of riverine DIC and the evasion of CO2 as the most likely mechanisms for 13C-enrichment in DIC, via isotope fractionation during HCO3- hydration and CO2 diffusion. Potentially, methanogenic activity could also be, at least partially, responsible for the 13C-enrichment in DIC, particularly immediately downstream of the Langat Reservoir, but due to the absence of empirical data, this must remain only a theoretical proposition. The aquatic chemistry and dissolved carbon data suggests that pollution discharge into the Langat and Kelantan Rivers is the major factor that is responsible for the considerable CO2 overpressures and high DIC and DOC concentrations in the river waters, particularly in the downstream sections. This pollution is likely of biological origin, via sewage and palm oil mill effluent (POME) discharge, and therefore isotopically indistinguishable from natural C3 plant sources. Carbon budgets of the Langat and Kelantan River show CO2 degassing to be a significant mechanism of fluvial carbon loss, comprising roughly 50 %, or more, of the total riverine carbon export in both watersheds. The remainder of the river carbon is transported to the ocean in the form of DIC, DOC and POC in broadly comparable proportions. However, the combined riverine carbon export from the Kelantan and Langat Basins amount to 2 % or less of the total carbon sequestration of the watersheds. Thus, most of the sequestered carbon is returned to the atmosphere via respiration, with smaller amounts incorporated into ecosystem biomass . These results highlight the complexity of carbon cycling in tropical rivers, and agree with previous studies in showing riverine systems to be more than simple conduits of carbon from the land to the ocean.

The reaction between aqueous Fe(II) and Fe(III) oxides is extremely complex, and can catalyze Fe(II)-Fe(III) electron transfer, exchange of Fe atoms between the aqueous and solid phases, mineral transformation, and contaminant reduction. Together, these processes represent a phenomenon referred to as Fe(II)-catalyzed Fe oxide recrystallization, which has been observed under controlled conditions in the laboratory for numerous Fe oxides. In the environment, Fe oxides are likely surrounded by organic carbon in various forms, but their potential to interfere with Fe(II)-catalyzed Fe oxide recrystallization, and its subsequent environmental relevance has not been well studied. The Fe(II)-catalyzed recrystallization of stable Fe oxides goethite and magnetite was studied in the presence of several environmentally relevant classes of organic carbon. For both goethite and magnetite, Fe(II)-catalyzed recrystallization continued relatively undeterred in the presence of electron shuttling compounds, natural organic matter isolates, and extracellular polysaccharides. Slight inhibition was observed when spent media from dissimilatory iron-reducing cultures was present, but only by sorbing a long-chain phospholipid to the oxides was significant inhibition observed. The lack of interference by organic carbon indicates that Fe(II)-catalyzed Fe oxide recrystallization is likely to be relevant throughout a wide range of environments, and represents a significant process with regards to the geochemical cycling of Fe atoms, a claim supported by evidence of Fe(II)-driven isotope mixing in real soils. The movement of atoms during Fe(II)-catalyzed Fe oxide recrystallization is not limited to just Fe however. Multiple trace elements have been shown to exchange between the aqueous and solid phases along with Fe during the Fe(II)-catalyzed recrystallization of Fe oxides. The effect of organic carbon, both sorbed to the oxide surface and coprecipitated with the oxide, on Fe(II)-catalyzed atom exchange and transformation of ferrihydrite was studied. Again, the presence of organic carbon did not appear to influence Fe atom exchange kinetics. It also did not appear to influence the rapid transformation of ferrihydrite to lepidocrocite. The presence of organic carbon does appear to ultimately have implications for mineral transformation, as over longer time periods it stabilized lepidocrocite, preventing its subsequent transformation to magnetite or goethite.

Lakes are increasingly being recognized as an important component of the global carbon cycle, yet anthropogenic activities that alter their community structure may change the way they transport and process carbon. This research focuses on the relationship between carbon cycling and community structure of primary producers in small, shallow lakes, which are the most abundant lake type in the world, and furthermore subject to intense terrestrial-aquatic coupling due to their high perimeter:area ratio. Shifts between macrophyte and phytoplankton dominance are widespread and common in shallow lakes, with potentially large consequences to regional carbon cycling. I thus compared a lake with clear-water conditions and a submerged macrophyte community to a turbid, phytoplankton-dominated lake, describing differences in the availability, processing, and export of organic and inorganic carbon. I furthermore examined the effects of increasing terrestrial carbon inputs on internal carbon cycling processes. Pelagic diel (24-hour) oxygen curves and independent fluorometric approaches of individual primary producers together indicated that the presence of a submerged macrophyte community facilitated higher annual rates of gross primary production than could be supported in a phytoplankton-dominated lake at similar nutrient concentrations. A simple model constructed from the empirical data suggested that this difference between regime types could be common in moderately eutrophic lakes with mean depths under three to four meters, where benthic primary production is a potentially major contributor to the whole-lake primary production. It thus appears likely that a regime shift from macrophyte to phytoplankton dominance in shallow lakes would typically decrease the quantity of autochthonous organic carbon available to lake food webs. Sediment core analyses indicated that a regime shift from macrophyte to phytoplankton dominance was associated with a four-fold increase in carbon burial rates, signalling a major change in lake carbon cycling dynamics. Carbon mass balances suggested that increasing carbon burial rates were not due to an increase in primary production or allochthonous loading, but instead were due to a higher carbon burial efficiency (carbon burial / carbon deposition). This, in turn, was associated with diminished benthic mineralization rates and an increase in calcite precipitation, together resulting in lower surface carbon dioxide emissions. Finally, a period of unusually high precipitation led to rising water levels, resulting in a feedback loop linking increasing concentrations of dissolved organic carbon (DOC) to severely anoxic conditions in the phytoplankton-dominated system. High water levels and DOC concentrations diminished benthic primary production (via shading) and boosted pelagic respiration rates, diminishing the hypolimnetic oxygen supply. The resulting anoxia created redox conditions which led to a major release of nutrients, DOC, and iron from the sediments. This further transformed the lake metabolism, providing a prolonged summertime anoxia below a water depth of 1 m, and leading to the near-complete loss of fish and macroinvertebrates. Pelagic pH levels also decreased significantly, increasing surface carbon dioxide emissions by an order of magnitude compared to previous years. Altogether, this thesis adds an important body of knowledge to our understanding of the significance of the benthic zone to carbon cycling in shallow lakes. The contribution of the benthic zone towards whole-lake primary production was quantified, and was identified as an important but vulnerable site for primary production. Benthic mineralization rates were furthermore found to influence carbon burial and surface emission rates, and benthic primary productivity played an important role in determining hypolimnetic oxygen availability, thus controlling the internal sediment loading of nutrients and carbon. This thesis also uniquely demonstrates that the ecological community structure (i.e. stable regime) of a eutrophic, shallow lake can significantly influence carbon availability and processing. By changing carbon cycling pathways, regime shifts in shallow lakes may significantly alter the role of these ecosystems with respect to the global carbon cycle.
Seen werden zunehmend als wichtige Komponente im globalen Kohlenstoffkreislauf anerkannt. Natürliche Veränderungen und anthropogene Aktivitäten beeinflussen die Struktur der Artengemeinschaft von Seen, was Auswirkungen auf den Transport und Umsatz von Kohlenstoff hat. Diese Arbeit konzentriert sich auf die Beziehung zwischen Kohlenstoffkreislauf und der Gemeinschaftsstruktur der Primärproduzenten in kleinen Flachseen. Diese sind der weltweit häufigste Seentyp und weisen durch ihren im Vergleich zur Fläche großen Umfang eine intensive aquatisch-terrestrische Kopplung auf. In Flachseen treten oft Regimewechsel zwischen Makrophyten- und Phytoplankton-Dominanz auf. Diese können potenziell große Konsequenzen für den regionalen Kohlenstoffkreislauf haben. In dieser Dissertation vergleiche ich einen Klarwassersee mit submersen Makrophyten und einen trüben, Phytoplankton-dominierten See hinsichtlich Verfügbarkeit, Umsatz und Export von organischem und anorganischem Kohlenstoff. Des Weiteren habe ich den Effekt der erhöhten Zufuhr von terrestrischem Kohlenstoff auf den internen Kohlenstoffumsatz untersucht. Sowohl die Tagesgänge der pelagischen Sauerstoff-Konzentrationen als auch Fluoreszenz-basierte Messungen der Primärproduktion bewiesen, dass die Präsenz von submersen Makrophyten eine höhere jährliche Brutto-Primärproduktion im Vergleich zu einem Phytoplankton-dominierten See mit ähnlichen Nährstoffkonzentrationen ermöglicht. Ein einfaches, auf den empirischen Daten basierendes Model zeigt, dass diese Unterschiede in der Brutto-Primärproduktion typisch sind für moderat eutrophe Seen mit einer mittleren Tiefe von unter 3 bis vier Metern. In diesen Seen leistet die benthische Primärproduktion den Hauptbeitrag zur Primärproduktion des ganzen Sees. Daraus wird ersichtlich, dass Regimewechsel von Makrophyten- zur Phytoplankton-Dominanz in Flachseen die Verfügbarkeit von autochthonem organischem Kohlenstoff für das Nahrungsnetz reduzieren. Paläolimnologische Analysen in Sedimentkernen beider Seen wiesen darauf hin, dass der Verlust der Makrophyten mit einer vierfachen Zunahme der Kohlenstoff-Speicherraten einhergeht, und somit zu einer großen Veränderung der Dynamik des Kohlenstoffkreislaufs im See führt. Unsere Kohlenstoff-Massenbilanzen zeigen, dass die Erhöhung der Kohlenstoff-Speicherung im Sediment nicht durch die Erhöhung der Primärproduktion oder durch externe Quellen, sondern durch erhöhte der Effizienz der Speicherung begründet war. Dies geht mit einer reduzierten benthischen Mineralisierungsrate und einer erhöhten Calcitfällung einher und führt zu reduzierten Kohlendioxid-Emissionen. Eine Periode ungewöhnlich hoher Niederschläge mit erhöhten Wasserständen führte im Phytoplankton-dominierten See zu zu einem starken Anstieg der Konzentrationen an gelöstem organischem Kohlenstoff (DOC) und zu anoxischen Bedingungen. Es wurde postuliert, dass zwischen diesen Prozessen eine positive Rückkopplung besteht. Die hohen Wasserstände und DOC-Konzentrationen reduzierten die Lichtversorgung und damit die Primärproduktion im Benthal und erhöhten die pelagischen Respirationsraten. Dadurch verringerte sich die Sauerstoffverfügbarkeit im Hypolimnion. Die dadurch erzeugten Redox-Verhältnisse führten zu einer Freisetzung großer Mengen an Nährstoffen, DOC und Eisen aus dem Sediment. Die während des gesamten Sommers andauernden anoxischen Verhältnisse in Wassertiefen unter 1 m führten zu einem fast vollständigen Verlust von Fischen und Makroinvertebraten. Zusätzlich wurde der pH-Wert im Pelagial signifikant erniedrigt und die Kohlenstoffdioxid-Emissionen im Vergleich zu früheren Jahren verzehnfacht. Insgesamt trägt diese Dissertation wesentliche Aspekte zum besseren Verständnis der Bedeutung des Benthals für den Kohlenstoffkreislauf in Flachseen bei. Der Anteil der benthischen Zone an der Primärproduktion in kleinen Flachseen wurde in Relation zur Gesamtproduktion des Systems quantifiziert. Letztlich zeigt diese Arbeit, dass die Gemeinschaftsstruktur der Primärproduzenten eines eutrophen Flachsees die Verfügbarkeit und den Umsatz von Kohlenstoff signifikant beeinflusst. Regimewechsel in Flachseen können durch Änderungen im internen Kohlenstoffkreislauf deren Rolle im globalen Kohlenstoffkreislauf verändern.

Previous pulse-chase experiments have revealed a wide diversity of benthic response patterns to organic matter (OM) input depending on environmental setting, benthic community structure and experimental conditions i.e. quantity and quality of the added OM.  However, the mechanisms and interaction of environmental and biological factors that produce an observed response pattern are poorly understood. The present thesis set out to improve our current understanding on the set of parameters that determine benthic response patterns.  The core of this study was based on two pulse-chase experiments in two bathyal settings: the Faroe-Shetland Channel (FSC) and the SW Cretan slope in the E. Mediterranean (E. Med).  The sub-zero temperatures in the FSC enabled the observation of the benthic response in “slow-motion” and showed that the response is not static but instead might go through various “phases”.  In the warm E. Med, C processing rates were considerably lower compared to previous measurements in adjacent regions.  The discrepancy was attributed to the particularly refractory sedimentary OM at the sampling station with apparent consequences for the physiological state of the bacterial community.  Both experiments showed that bacterial metabolism and its regulation is a key factor determining the reaction of the benthic community to OM inputs.  This thesis provided further understanding on the short-term fate of organic C in deep-sea sediments but also raised certain issues that could be addressed in future studies.

Thesis: S.B., Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, 2018.
Cataloged from PDF version of thesis.
Includes bibliographical references (page 18).
Soil is Earth's largest terrestrial carbon pool (Oertel et al., 2016) and can act as a net source of greenhouse gases (GHG). However, if organic material accumulates in soils faster than it is converted to CO2 by cellular respiration, soil becomes a smaller GHG source and even has the potential to become a GHG sink. Not much is known about factors that drive soil to be a source or a sink of GHG. Soil temperature and moisture have both been shown to correlate with CH4 emissions and temperature has been shown to correlate with CO 2 emissions (Jacinthe et al., 2015). Currently these relationships are not well constrained, particularly in upland soils, which are soils found at elevations between 100 and 500 m (Carating et al., 2014). Soil from the Harvard Forest was collected and used in two in-lab flux experiments to constrain the effect that soil moisture has on i.) the rate of CH4 and CO2 production/consumption and ii.) the fraction of injected CH4 that is oxidized to CO2 by soil microbes. The first experiment involved injecting vials containing soil samples with CH4 , taking an initial measurement with a residual gas analyzer (RGA), incubating for three days, and taking final measurements using the RGA. The results of this experiment indicated that cellular respiration is an important carbon source in these soils, with more CO2 coming from cellular respiration than from the oxidation of CH4. The second experiment involved injecting vials containing soil samples with CH4 and 14CH4 as a tracer, incubating for six days, and analyzing CO2 from each sample using a scintillation counter. This experiment showed a weak trend indicating that increased soil moisture may result in decreased CH4 oxidation. Results showed that decays per minute from the samples were lower than in a control. These results indicated that not all CO 2 from each sample was successfully captured and analyzed using the methods here. So while the trend may hold true, it should be supported by reconducting the experiment using a more reliable means of CO2 capture. The unexpected results from both experiments indicated that there is still much to be learned about the reactions that occur in these soils and how to perfect laboratory methods to study them.
by Alexa Jaeger.
S.B.

Peatlands are one of the largest global terrestrial carbon (C) pools, and play a vital role in provision of key ecosystem functions and as refugia for biodiversity. Many peatlands continue to be exploited with lowland raised bogs among the most affected by human modification. It is also now recognised that global climate change has potential to cause further impacts to peatlands, and it is thought that northern peatlands are particularly vulnerable to changes in temperature and precipitation. In this thesis, I report from a series of experiments to test; 1) the effect drought on soil CO2 efflux and photosynthate allocation, and production and chemical composition of dissolved organic carbon in leachate, 2) the effects of ericoid mycorrhizal (ERM) fungal necromass on soil CO2 efflux, 3) whether nitrogen (N) from ERM fungal necromass is important for plant nutrition, and 4) how different species of ERM fungi affect C and N turnover. These experiments were undertaken using a combination of field manipulations and measurements, and establishment of simplified mesocosms and microcosm systems. My results show that soil CO2 efflux in lowland degraded peatland is driven by the depth of water table, and that management of these systems from a C cycling perspective should consider ways to stabilise water table depth. Interpretation of data from field-girdling of C. vulgaris plants and 13CO2 pulse labelling strongly suggested that recent plant photosynthate has little apparent effect on this flux in contrast to many other ecosystems. Although the biomass of ERM fungi is often assumed to have a minor role in C cycling, my data show that the necromass of these fungi is highly labile and turnover rapidly, with potential to make important contributions to CO2 efflux and other microbially-driven processes.

A 3-year study was carried out on the role of microzooplankton in carbon cycling in the south Atlantic and the Atlantic sector of the Southern Ocean. Microzooplankton grazing impact on phytoplankton was estimated during austral summer and winter employing the dilution technique. Carnivory by larger zooplankton on microzooplankton during summer was estimated using in vitro incubations. Microzooplankton assemblages were always dominated by protozoans comprising ciliates and dinoflagellates. In the ( 20 um chlorophyll fraction, microzooplankton grazing was sufficient to control the growth of the nano- and picophytoplankton suggesting that, where larger microphytoplankton cells dominate, micro zooplankton maintain the background concentrations of the nano- and picophytoplankton. During winter, when small nano- and picophytoplankton cells dominate total chlorophyll concentrations, the microzooplankton grazing impact on phytoplankton is dramatically increased. Microzooplankton removed on average 37% of the initial phytoplankton stock or 70% of the daily phytoplankton production. These results suggest that in winter, micro zooplankton are the main sink for phytoplankton production. Carnivory experiments conducted with selected meso- (copepods) and macro zooplankton (euphausiids and tunicates) showed that all species examined consumed micro zooplankton in the presence of substantial chlorophyll concentrations. Microzooplankton can, therefore, be regarded as trophic intermediates between bacterioplankton, small phytoplankton cells and larger zooplankton species in the Southern Ocean. The results of this investigation suggest a spatiotemporal shift in efficiency of the biological pump mediated by changes in the size composition of the phytoplankton assemblages. South of the Antarctic Polar Front (APF) large IV microphytoplankton cells dominate the summer chlorophyll biomass, suggesting that larger zooplankton grazers represent the main sink for phytoplankton production. Under these conditions, carbon flux to the interior of the ocean will be high due to diel vertical migrations by grazers and the production of large, fast sinking faecal pellets. The sedimentation of large phytoplankton cells also contributes to flux. In the permanently open waters south of the APF and throughout the Southern Ocean during winter, small phytoplankton cells dominate total chlorophyll, resulting in the microbial loop being the main sink for phytoplankton production. The close coupling between the micro zooplankton and the microbial loop dramatically reduces the transfer of organic carbon from the surface layers to depth. Carnivory by metazoans on microzooplankton may reduce the high grazing impact of micro zooplankton and, may also represent an important source of carbon flux originating from the microbial loop.

Nitrogen deficiencies have long been acknowledged as a factor limiting the restoration of ecosystems destroyed by surface mining in the Appalachian Region of the U.S. The fundamental ecological structure and function common to intact terrestrial ecosystems are largely lacking in mine soils. Reliable guidelines for effective long-term restoration require a detailed understanding of the ecological processes occurring within the mine-soil system. The objective of this study was to determine the extent to which inorganic N fertilization, native topsoil replacement, or whole-tree wood-chip amendment affected the restoration and reforestation of an Appalachian mine-soil system through changes in C and ~ dynamics. Eighteen concrete tank lysimeters filled with mine spoils served as experimental microcosms to test hypotheses set forth in this study. Treatment effects on soil N and C pools, herbaceous biomass production, N uptake, N fluxes between pools, net leachate N losses, and early growth of pitch x loblolly hybrid pines were evaluated at regular intervals between July 1987 and October 1989. Inorganic N fertilization increased aboveground herbaceous biomass yield and N uptake by 87 and 71%, respectively, during the first growing season, but did not significantly affect yield or N uptake thereafter. During the first growing season, biomass production was 38% higher in the topsoil-amended mine soil than the unamended control. This resulted in an additional 17.4 kg N ha-1 sequestered in comparison to the control. Biomass yield was 270 and 19% lower in the wood-chip-amended mine soil than the unamended control after the first and third growing seasons, respectively. This resulted in 63 and 25% less N uptake, respectively, than the control. Survival of pitch x loblolly pine after two growing seasons was 90% in the N-fertilized mine soil and 71% with the fertilizer control treatment. This difference in survival was the result of lower water potential in the unamended mine soil during the growing season in which the trees were planted. Nitrogen fertilization did not significantly affect tree growth or nutrition. Pine survival after two growing seasons was 83, 98, and 60% for the unamended control, wood-chip, and topsoil treatments, respectively. By the end of the second growing season, the wood-chip treatment also resulted in greater tree height, ground-line diameter, and stem-volume index by 30, 49, and 203% respectively, when compared to the control. Increased survival and growth in the wood-chip-amended mine soil were directly related to higher soil water potential than the control or topsoil treatments. Total inorganic N leaching loss from N-fertilized mine soil was 47.64 kg ha-1 yr-1 higher than the control during the first growing season. However, N fertilization losses were not significantly higher during the remainder of the study period. Drainage was significantly higher during all three growing seasons in the wood-chip-amended mine soil. This resulted in lower N sequestering during the third growing season when precipitation was most abundant. Topsoil amendment did not significantly affect N leaching losses. Inorganic N fertilization did not significantly affect total organic C, total N, or N availability indices in the mine soil. Following topsoil addition, mine-soil total N was 294% higher than the unamended control. Wood-chip effects on the soil organic-matter pool were more gradual; however, by the end of the study, total N and total organic C were 18 and 95% higher, respectively in the wood-chip-amended mine soil than in the unamended control. Aerobic incubation of soil samples collected near the end of the second growing season showed that the topsoil and wood-chip amendments increased the N mineralization potential by 101 and 55%, respectively, in comparison to the unamended control. Furthermore, the mineralization rate constant of the wood-chip-amended mine soil was 44% lower than the control. This shows a slower rate of N turnover and more stable mine-soil N pool with the wood-chip treatment. This study shows that inorganic N fertilizer effects on N and C dynamics were rapid but transient. In contrast, the surface-applied amendments of native topsoil and whole-tree wood chips improved the potential for successful restoration of forests by increasing the N cycling capacity of the developing mine-soil system.
Ph. D.

Geochemical analyses of Holocene coastal sediments from eastern England were made to better understand the cycling of organic carbon and nutrients in the coastal zone in the past, present and future. Sediments and peat were deposited in freshwater marshes, saltmarshes and intertidal mud- and sand-flat environments that were much more extensive during the Holocene than they are at present. The reduction in these areas, largely through human activities, has decreased the potential annual accumulation and storage of organic carbon, nitrogen and phosphorus associated with sediments. While the carbon and nitrogen contents of modem intertidal environments are similar to Holocene intertidal areas, phosphorus is enriched in modem sediments by up to a factor of two. Budgets of nitrogen and phosphorus cycling in Fenland, eastern England, suggest that the Holocene estuaries in this area were sinks of nutrients from the North Sea despite nitrogen isotopic evidence suggesting that nitrogen buried in freshwater marshes was predominantly terrestrially derived. The present-day estuaries are sources of nutrients to the North Sea as riverine loads and atmospheric deposition are much higher than during the Holocene and sedimentation is also greatly reduced. The southern North Sea is probably autotrophic, in contrast to the coastal zone global average which is heterotrophic. The major differences between these two areas are: 1) the global coastal zone receives much greater loads of riverine particulate matter than the southern North Sea, and 2) sedimentation in the global coastal zone occurs in large river deltas which are absent from the relatively small European estuaries, thus much of the sediment supplied to the North Sea is exported to the shelf edge. Approximately 4x 109 t C, 0.3 x 109 tN and 0.1 x 109 tP are currently stored in fine-grained Holocene sediments in the southern North Sea coastal zone.

Thesis: Ph. D., Joint Program in Chemical Oceanography (Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2016.
Cataloged from PDF version of thesis.
Includes bibliographical references.
Characterizing global and regional climate variability and climate-carbon cycle interactions in the past provides critical context for evaluating present and future climate trends. In this thesis, I use stable isotope and radiocarbon analysis of vascular plant biomarkers in lacustrine and marine sediment cores to explore late Quaternary climate variability and connections between past climate change and terrestrial carbon cycling in tropical South America. I investigate temporal and spatial trends in South American Summer Monsoon precipitation by reconstructing hydrologic variability over the past 50,000 years at two sites: the Lake Titicaca drainage basin in the Central Andes and the Pantanal wetlands in the interior lowlands. Diverging hydrologic trends at these two sites during the last glacial period suggest altered monsoon circulation patterns under glacial conditions, while changes in summer insolation appear to be an important control of precipitation at both sites during the Holocene. I next assess the relationship between climate change and the age structure of terrestrial biospheric carbon exported from two tropical catchments over the past 20,000 years. Radiocarbon dating of leaf waxes in Cariaco Basin and Lake Titicaca sediment records indicates that waxes preserved in sediments are likely composed of a fresh component transported to sediments within decades of production by vegetation and an old component derived from aged soil organic matter with an average age on the order of millennia at time of deposition. Results from both sites show that past hydrologic variability had a significant impact on the mobilization and export of different pools of terrestrial biospheric carbon. In particular, results from Cariaco Basin suggest that wetter conditions in the past resulted in increased export of fresh biospheric carbon to the ocean, representing a potentially important climate feedback mechanism on geologic timescales.
by Kyrstin L. Fornace.
Ph. D.

The end-Permian extinction at 252 Ma is widely regarded as the most severe of the Phanerozoic mass-extinctions and enabled the evolution of the modern carbon cycle and ecosystem structure. The cause of the extinction is still debated but the synergistic pressures of global climate change, such as anoxia and ocean acidification, were clearly important. The extinction occurred in two phases and is marked by a uniquely protracted recovery period of ~ 5 Myrs where diversity fails to reach pre-extinction levels until the Middle Triassic. This period is characterized by an unstable global carbon cycle, secondary extinctions, reef, chert and coal gaps, and changes in the carbonate factory from reef to microbial and abiotic dominated deposition. This thesis focuses on using geochemical data from the Arabian Margin to investigate the carbon cycle record and the links between kill mechanisms and carbon cycle dynamics. A new record of carbon cycling is presented for the Tethys in the form of a carbon isotope record for the entire Early Triassic from the Musandam Peninsula, United Arab Emirates (UAE). The Musandam carbon isotope record can be broadly correlated with global isotopic events but also resolves additional secondary excursions. These new short-lived events are probably related to the occurrence of the more widely recognized Early Triassic excursions, and may represent fluctuations in the driving mechanisms superimposed on the continued instability of the global carbon cycle in the aftermath of the end-Permian extinction. To unravel palaeo-depositional redox conditions this work utilizes geochemical proxies based on Fe systematics (Fe-speciation). To date, however, these proxies have only been calibrated in relation to modern and ancient siliciclastic marine sediments. This clearly limits the use of the Fe-speciation proxy, particularly in relation to carbonate-rich sediments and rocks. This thesis explores the use of Fe-speciation in carbonates using compiled literature and new data from modern oxic and anoxic settings. This new assessment expands the utility of Fe-based redox proxies to also incorporate carbonate-rich rocks that contain significant total Fe (>0.5 wt%), providing care is taken to assess possible impacts of diagenetic processes such as dolomitization. Based on this calibration work Fe-speciation is used to reconstruct the redox structure for the Arabian Margin mixed carbonate and clastic sediments, from the late Permian to the Middle Triassic. Fe-S-C systematics are utilized to identify the spatial and temporal dynamics of anoxia for a Neo-Tethyan shelf-to-basin transect. The unique spatial resolution afforded by this transect allows a direct link to be drawn between biodiversity, carbon cycling and anoxic events. For the first time we can directly observe a switch from deep-ocean dominated anoxia to a dynamic anoxic wedge at the end-Permian extinction. Additionally the data suggest that ferruginous conditions (anoxic non-sulphidic) were dominant in the Tethyan Ocean throughout the Early Triassic, proposing that euxinia was restricted regionally with potential implications for nutrient recycling, carbon cycle models and driving mechanisms. Redox dynamics may have had important implications for the wider carbonate cycle. These two themes are particularly inter-related with regards to oceanic alkalinity and pH. This thesis presents the first shallow water boron isotope record for the Permian Triasssic Boundary, used as a proxy for pH. The record demonstrates some unexpected results; firstly a sudden increase in pH is observed, prior to the first phase of the extinction and interpreted to reflect alkalinity supply from the development of slope anoxia. Secondly there is no evidence for an acidification event at the first phase of the extinction where pH remains stable. A rapid acidification event is, however, seen in the earliest Triassic, contemporary to the second phase of the mass extinction, but delayed compared to the main negative carbon isotope excursion that indicates the main phase of Siberian Trap volcanism. These events may be explained by dramatic changes in ocean the ocean’s buffering capacity linked to changes in alkalinity supply and the carbonate factory.

The mid-Cretaceous (Aptian–Cenomanian) climate was characterised by steadily increasing temperatures likely driven by high atmospheric CO2. The climate system was dynamic: throughout this interval there were several dramatic carbon cycle perturbations (of 1–2 Myrs duration) due to initiation of marine anoxia (OAEs) resulting in burial of organic carbon. However, pCO2 values and trends are generally poorly constrained for much of this time interval. During the mid-Cretaceous, angiosperms (flowering plants) underwent a rapid poleward diversification and radiation; by the Cenomanian they comprised around 70% of floras. However, hypotheses detailing the competitive replacement of incumbent floras by advantageous angiosperm adaptations do not fully explain the timing and nature of early angiosperm evolution. This thesis provides a record of Albian–Cenomanian carbon cycling and explores the role of climate change and pCO2 decline (CO2 starvation hypothesis) as forcing factors on angiosperm radiation. This is achieved using fossil material from the Nuussuaq Peninsula, West Greenland. Carbon isotope stratigraphy constrains the stratigraphic age (Middle Albian-Cenomanian) and identifies two intervals of carbon cycle disturbance. Macerated leaf cuticle and palynological studies reveal detailed floral assemblages (in which angiosperms, including Eudicots, were poorly represented but present throughout) and unprecedented ecological information. New pCO2 estimates for the Middle Albian are generated from stomatal density measurements, which, integrated with other similar datasets, suggest average pCO2 in the Aptian-Early Cenomanian of 575 ppm with a decline of ~150 ppm in the Middle Albian. The subsequent rise in pCO2 through to the Late Albian coincides with a 30 % increase in angiosperm abundance and increased global temperatures; strongly suggesting the role of climate on angiosperm radiation. However, comparisons of vein density, stomatal conductance, stomatal density and pore length between fossil and extant angiosperms reveals angiosperms already possessed advantageous adaptations expected from a low pCO2 climate by the mid-Albian.

Farmers and land managers can yield financial gains through the sale of carbon credits. They typically rely on computationally complex models fit using sparse datasets to make predictions concerning soil carbon stocks and associated carbon accounts which can lead to over-fitting. We develop new soil carbon models to address the over-fitting issue and evaluate the effect of microbial biomass on modelling soil carbon sequestration. Also, we introduce and evaluate advanced Bayesian methods to improve the speed of computation and accuracy of predictions of soil carbon stocks.

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The aim of this study was to evaluate the effects in the Caatinga biome of the technique of thinning on the dynamics of herbaceous phytomass productivity; fine roots; the deposition, accumulation and rates of decomposition of litter; aggregate stability; respirometry of the soil; stocks of carbon and nitrogen; and characterisation of changes in the spectral behaviour of the canopy. The study was carried out in two stages: the first in watersheds in the town of Iguatu, in the south central region of the State of CearÃ, Brazil, and the second in the watershed of the Caxitorà River, in the northern region of the state. The Iguatu experimental area comprised two watersheds, one under thinning for 5 years (CR5) and the other preserved with natural vegetation for 35 years (CS35). The variables to be sampled were: productivity of herbaceous biomass; fine roots; gravimetric moisture; isotope δ13C (â); aggregate stability; deposition, accumulation and rates of decomposition of litter; and stocks of total organic carbon and total nitrogen in the 0-20, 20-40 and 40-60 cm layers, from April 2013 to March 2014. The data were submitted to the analysis of means test and compared by t-test (p≤ 0.05). The increase in the intensity of herbaceous plants resulting from thinning contributed to an increase in stocks of total organic carbon (249% and 139% in the 20-40 cm and 40-60 cm layers respectively) and of total nitrogen (142% and 137% respectively), in relation to the area under preservation. The 0-10 cm layer of CR5 stored double the amount of fine roots found in CS35. In the topsoil (0-20 cm) of CR5, aggregates with a size 2.15 times greater than those found in CS35 were obtained. The greater conservation of litter on the soil of CR5 is associated with the greater input of lignified woody biomass from thinning, and consequently with the lower rates of decomposition and respirometry, which are evidenced by the low CO2 emissions into the atmosphere. Thinning in a Vertisol of the Caatinga biome has an influence on soil structure and on the stocks of carbon and nitrogen, making possible lower rates of CO2 emission and improving conditions for the infiltration of water. In the second phase of the experiment, SRTM (Shuttle Radar Topography Mission) data were used to evaluate the effect of the illumination geometry on the spectral characterisation of the canopy, as well as images from the OLI/Landsat 8 sensor, representative of the rainy and dry seasons of 2013. The images were converted into physical values (surface reflectance factors), the NDVI was calculated, and with the technique of principal component analysis images PC1 and PC2 were generated. Dispersion for the values of PC1 and PC2 from the different canopies was evaluated in a two-dimensional space. It was found that the reflectance intensity of the incident electromagnetic radiation in canopies of the caatinga biome is not only influenced by seasonality, but also by the illumination geometry arising from the topographical characteristics of the terrain. The effect of shading was predominant during the dry season, especially under low lighting conditions, irrespective of the structure of the plant cover. The NDVI proved to be unsuitable for detecting changes in the spectral behaviour of the Caatinga biome during the rainy season.
Objetivou-se com este trabalho avaliar os efeitos da tÃcnica de raleamento do bioma caatinga sobre a dinÃmica da produtividade de fitomassa herbÃcea; raÃzes finas; deposiÃÃo, acumulaÃÃo e taxas de decomposiÃÃo da serapilheira; estabilidade de agregados; respirometria do solo; estoque de carbono, nitrogÃnio e caracterizaÃÃo das alteraÃÃes no comportamento espectral do dossel. O estudo foi conduzido em duas etapas: a primeira em microbacias hidrogrÃficas no municÃpio de Iguatu, regiÃo centro sul e a segunda na bacia hidrogrÃfica do rio CaxitorÃ, regiÃo norte, ambas no estado do CearÃ. A Ãrea experimental de Iguatu consistiu de duas microbacias adjacentes, sendo uma raleada por 5 anos (CR5) e a outra conservada com vegetaÃÃo natural hà 35 anos (CS35). As variÃveis amostradas foram: produtividade de fitomassa herbÃcea, raÃzes finas, umidade gravimÃtrica, isÃtopo δ13C (â), estabilidade de agregados, deposiÃÃo, acumulaÃÃo e taxas de decomposiÃÃo da serapilheira, estoque de carbono orgÃnico total e nitrogÃnio total nas camadas de 0-20, 20-40 e 40-60 cm entre abril/2013 e marÃo/2014. Os dados foram submetidos à anÃlise de teste de mÃdia e confrontados pelo Teste T (p≤0,05). O aumento da intensidade de plantas herbÃceas decorrente da tÃcnica de raleamento contribuiu para o acrÃscimo nos estoques de carbono orgÃnico total (249% e 139% nas camadas 20-40 cm e 40-60 cm, respectivamente) e do nitrogÃnio total (142% e 137%, respectivamente) em relaÃÃo à Ãrea conservada. A camada de 0-10 cm da parcela CR5 armazenou o dobro do quantitativo de raÃzes finas encontradas em relaÃÃo a CS35. Na camada superficial do solo (0-20 cm) da CR5 foram obtidos agregados com tamanho de 2,15 vezes acima dos obtidos na CS35. A maior conservaÃÃo de serapilheira no solo da CR5 està associada aos maiores aportes de fitomassa lenhosa lignificada proveniente do raleamento, e consequentemente das menores taxas de decomposiÃÃo e respirometria, constatada pelas baixas emissÃes de CO2 à atmosfera. O raleamento em Vertissolo do bioma Caatinga exerce influÃncia na estruturaÃÃo do solo, no estoque de Carbono e NitrogÃnio, possibilitando as menores taxas de emissÃo de CO2 e melhorando as condiÃÃes para a infiltraÃÃo de Ãgua. Jà na segunda etapa do experimento, foram utilizados dados do SRTM (Shuttle Radar Topography Mission) para avaliaÃÃo do efeito da geometria de iluminaÃÃo na caracterizaÃÃo espectral do dossel, alÃm de imagens do sensor OLI/LANDSAT 8 representativas da estaÃÃo chuvosa e seca de 2013. As imagens foram convertidas em valores fÃsicos (fatores de reflectÃncia de superfÃcie), calculado o Ãndice NDVI e, a partir da tÃcnica de componentes principais, geradas imagens PC1 e PC2. Foram avaliadas as dispersÃes dos valores de PC1 e PC2, em um espaÃo bidimensional, provenientes dos diferentes dossÃis. Os resultados indicaram que a intensidade de reflectÃncia da radiaÃÃo eletromagnÃtica incidente em dossÃis do bioma caatinga à influenciada tanto pela sazonalidade climÃtica quanto pela geometria de iluminaÃÃo decorrentes das caracterÃsticas topogrÃficas do relevo. O efeito sombreamento foi predominante durante o perÃodo seco, principalmente para condiÃÃo de pouca iluminaÃÃo independentemente da estrutura de cobertura vegetal. O NDVI nÃo se mostrou adequado para detectar alteraÃÃes no comportamento espectral do bioma caatinga durante a estaÃÃo chuvosa.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 2008.
Includes bibliographical references.
Emissions of black carbon (BC), the soot and char formed during incomplete combustion of fossil and biomass fuels, have increased over the last century and are estimated to be between 8 and 270 Tg BC/yr. BC may affect problems as diverse as global warming, human health, carbon cycling in ecosystems, and pollutant dynamics. However, currently there is substantial uncertainty with respect to the fate of BC released to the environment. To increase our understanding of BC's fate and effects, modifications to the Chemo-Thermal Oxidation method at 375 'C (CTO 375), a current method used to quantify BC in marine sediments, were made to measure BC concentrations in, and fluxes out of, the water column in the Gulf of Maine (GoM), a representative coastal area downwind of important BC sources of the Northeastern United States. In addition, an alternative method to infer BC concentrations in seawater by observing pyrene fluorescence losses (PFL) in spiked samples was developed. Average concentrations measured in the GoM were 5 and 4 [tg/L using the modified CTO 375 and PFL methods, respectively. Although these two methodologies involve independent observations, correspondence between the modified CTO 375 and PFL methods suggested that the isolated material was both highly sorptive and refractory. These concentrations also suggested that (a) up to 50% of the "molecularly uncharacterized" particulate organic carbon (POC) in surface seawater is BC; (b) the presence of this recalcitrant organic carbon may explain why some POC is not recycled to CO2 during its transport to depth and even within the sediment beds below; and (c) hydrophobic pollutants like polycyclic aromatic hydrocarbons (PAHs) and dioxins would have their "bioavailabilities" controlled by sorption to BC.
(cont.) The observed BC spatial distributions and average water-column export fluxes near 10 gBC/m2 year imply that most of the BC is carried offshore by wind and is accumulated in the coastal zone. Finally, sediment records of CTO-375-derived BC inputs into the GoM for the past 100 years were consistent with historical changes in fossil and biomass fuel emissions to this area.
by Déborah Xanat Flores-Cervantes.
Ph.D.

During the fourth International Polar Year, an interdisciplinary study was conducted to examine the couplings between sea ice, ocean, atmosphere, and ecosystem in the southeastern Beaufort Sea. This thesis examines components of the system that control the air–sea exchange of carbon dioxide. Using eddy covariance measurements, we found enhanced CO2 exchange associated with new ice formation in winter flaw leads. This exchange was typically directed towards the surface, although we also measured one instance of outgassing. Sea surface dissolved CO2 measurements (pCO2sw) in Amundsen Gulf showed significant undersaturation with respect to the atmosphere at freeze–up, followed by a slow increase over the winter until spring phytoplankton blooms caused strong undersaturation at break–up. Over the summer, pCO2sw increased until becoming slightly supersaturated due to surface warming. Along the southern margins of Amundsen Gulf and on the Mackenzie Shelf we found pCO2sw supersaturations in the fall due to wind–driven coastal upwelling. In the spring, this upwelling occurred along the landfast ice edges of Amundsen Gulf. By combining observations of enhanced winter gas exchange with observations of pCO2sw in Amundsen Gulf, we derived an annual budget of air–sea CO2 exchange for the region. This exercise showed that uptake through the winter season was as important as the open water season, making the overall annual uptake of CO2 about double what had previously been calculated. Prior to this work, the prevailing paradigm of air–sea CO2 cycling in Arctic polynya regions posited that strong CO2 absorption occurs in the open water seasons, and that a potential outgassing during the winter is inhibited by the sea ice cover. As a new paradigm, we propose that the spatial and temporal variability of many processes – including phytoplankton blooms, sea surface temperature and salinity changes, upwelling, river input, continental shelf processes, and the potential for high rates of winter gas exchange – need to be considered in order to understand the carbon source/sink status of a given Arctic polynya region. A paradigm that considers such varied processes is useful in understanding how climate change in the Arctic can impact air–sea CO2 exchange.

Over the past few decades, there has been increased research focus on carbon cycling within aquatic systems, especially with the changing global climate. Inland waters play a major role in the global carbon cycle, but the fundamental features remain poorly understood, particularly the large lakes of the world. Our experimental approach assessing the carbon budget of Lake Superior, the largest freshwater lake by area, provides spatial and temporal variability that has been previously overlooked but may be critical to our understanding on the biogeochemical processes controlling the lake. Multiple stations were chosen across the lake, both nearshore and offshore, to evaluate the variability in physical mixing regimes and biogeochemical processing. Short and long-term carbon consumption measurements were coupled to assess the heterotrophic activity relative to the lability of dissolved organic carbon. Partial pressure of carbon dioxide (pCO2) was directly measured to determine the metabolic nature of the lake and the amount of carbon dioxide (CO2) that fluxes across the air-water interface. The pCO2 results were further coupled with an isotopic approach measuring oxygen-18 (δ18O) to evaluate how the metabolism of Lake Superior has changed over a decadal scale. A range of environmental factors, including temperature, photodegradation and source/quality of organic carbon, influenced short and long-term carbon consumption. In-situ pCO2 observations supported a temporal switch in metabolism from the lake being a source of CO2 in the spring to being a sink in the summer driven by biological components of the system. When the pCO2 results were coupled with the isotopic measurements over the past decade (1999-2011), Lake Superior was dominated by respiration during isothermal conditions and production during stratification. In the past decade, Lake Superior has experienced increased surface water temperatures, shifting the metabolic state to a shorter net heterotrophic period in the spring and a longer net autotrophic period in the summer. This research highlights fundamental aspects of Lake Superior’s metabolism that have been previously understudied, as well as providing key information about processes controlling its carbon budget, and giving a better understanding of how climate change will continue to impact Lake Superior.

Inland waters have long been neglected in the global carbon cycle. They represent only 2,8% of the land area, but it has come clear that inland waters play a key role in the transformation of terrestrial fixed carbon to the atmosphere. Human activities do have an impact on the carbon cycling and it is important to understand how these changes affects natural biogeochemical and climatological processes. The purpose of this report was to investigate how forestry impacts the emission of carbon dioxide from boreal lakes and to evaluate which role lakes play in the global carbon cycle. The study was accomplished as a literature study and the search words that have been used are carbon cycling, carbon dioxide, forestry, boreal lakes, dissolved organic carbon and pCO2. The results show that in many studies does forestry increase the export of dissolved organic carbon from terrestrial environments to boreal lakes. This increase subsidies the net heterotrophy in boreal lakes, making them net sources of carbon dioxide to the atmosphere. The processes behind increased concentrations and emissions are however complex and factors like local topography, hydrology and climate are thought to have impacts on how much carbon dioxide that is produced at a given level of dissolved organic carbon. Forestry seems to have an increasing effect on the carbon dioxide emissions, but the key drivers behind this process are expressed differently between regions and the reasons underlying these differences remain to be explored in order to make precise global carbon models.

Thesis (Ph. D.)--Earth and Atmospheric Sciences, Georgia Institute of Technology, 2007.
Martial Taillefert, Committee Member ; Jay Brandes, Committee Member ; Markus Huettel, Committee Member ; Philip Froelich, Committee Member ; Ellery Ingall, Committee Member ; Richard A. Jahnke, Committee Chair.

In the savanna woodlands of Southern Africa, locally know as miombo, carbon cycling is poorly quantified and many of the key processes remain obscure. For example, seasonal constraints on productivity and leaf display are not well understood. Also, fire is known to be a key process, with around 50% of the annual global area burned occurring in Africa, but detailed understanding of its ecological effects is lacking. Land use change and woodland degradation are changing the structure and functioning of these tropical woodlands, which cover 2.7 million km2 of Southern Africa and provide ecosystem services which support the livelihoods of over 100 million people. In this thesis I quantify the major carbon stocks of the woodlands in Nhambita Regulado, Gorongosa District in Sofala Province, Mozambique. I also examine processes that affect these stocks, including fire and clearance for agriculture. Furthermore, I quantify the seasonal cycle of leaf display, and its relationship to climate. I conducted a series of experimental burns and found that fire intensity was strongly related to rates of top-kill and root stock mortality. Top-kill rates decreased as tree diameter increased up to 10 cm DBH. After this point increased size did not affect top-kill rates, possibly because of accumulated wounds and rottenness. I then extrapolated these results to long term predictions of tree populations and carbon stocks by modelling the interactions of fire, mortality and tree growth. The model was able to successfully predict woody vegetation structure at two sites with known fire regimes, including a 50-year fire experiment in Marondera, Zimbabwe. The results show that annual fires in miombo suppress all woody vegetation. Low intensity fires every 2.5 years are required to maintain observed stem biomass in Nhambita. High intensity fires lead to high top-kill rates (12%), even among large stems. Manipulating fire intensity rather than frequency seems to be the most practical approach to limiting degradation by fire in these ecosystems. Using a three year time series of hemispherical photographs of the tree canopy, combined with satellite data, I find that tree leaf phenology is not directly related to seasonal rainfall patterns, both in Nhambita and across Southern Africa. Pre-rain green-up is the dominant phenology, from the semi arid savannas of the south of the continent to the wet miombo of the Congo basin. Wet miombo woodlands have longer periods of green-up before rain onset (mean 60 days) compared with dry miombo (37 days). Green up-dates show little interannual variability but large spatial variability. The importance of pre-rain green-up in determining how these ecosystems will respond to changing rainfall patterns is unknown, but is an important area for future study. I quantified carbon stocks in the Nhambita woodlands in the soil (69% of total carbon stocks of 111 tC ha-1), tree stems (19%) and roots (8%) as well as other smaller pools. An allometric relationship between root and stem biomass and stem diameter was developed, and used to evaluate the uncertainties in stem carbon estimation at plot and landscape scale. We find that the uncertainty (95% confidence intervals) at plot scale can be quite large (60% of the mean) but this is reduced to around 25% at landscape scale. Strategies for effective inventories of miombo woodland are presented. Using a chronosequence of abandoned farmland, we estimate that stem biomass recovers from clearance after around 30 years of abandonment. Changes in soil carbon stocks are less well understood and need further work. This thesis concludes by outlining further work needed to model the carbon cycle of these woodlands, as well as discussing the implication of pre-rain green-up for satellite observations of land cover changes and biomass mapping.

This study investigates how the estimated thickening of the active layer will affectthe soil organic carbon in permafrost soils. The focus lies on estimating how muchof the upper permafrost soil organic carbon will be affected by the active layerdeepening due to global warming, on what timescale the deepening will take placeand if the estimated changes differ depending on the extent of permafrost in theregion. A model made in a Geographic Information System (GIS) combines datasetsfrom The Northern Circumpolar Soil Carbon Database, field data of soil organiccarbon content (SOCC) in different permafrost soil horizons in the Usa basin anddata of recent and future active layer depth from a spatially distributed permafrostdynamics model in the Pechora River Basin. The model shows that in 1980, 75% ofthe available 0–100 cm Gelisol soil organic carbon mass (SOCM) has affected byseasonal thawing. In 2050 the proportion is increased to 86% and by 2090 almostthe whole study area has an active layer deeper than 1 meter (98%). This indicatesan increase from approximately 0.64% to 0.84% of the total 1–100 cm SOCM in thenorthern permafrost region. The change is more gradual in the isolated and thesporadic permafrost zones and more abrupt in the continuous and discontinuous regions.

Cropland management practices have traditionally focused on maximising the production of food, feed and fibre. However, croplands also provide valuable regulating ecosystem services, including carbon (C) storage in soil and biomass. Consequently, management impacts the extents to which croplands act as sources or sinks of atmospheric carbon dioxide (CO2). And so, reliable information on cropland ecosystem C fluxes and yields are essential for policy-makers concerned with climate change mitigation and food security. Eddy-covariance (EC) flux towers can provide observations of net ecosystem exchanges (NEE) of CO2 within croplands, however the tower sites are temporally and spatially sparse. Process-based crop models simulate the key biophysical mechanisms within cropland ecosystems, including the management impacts, crop cultivar, soil and climate on crop C dynamics. The models are therefore a powerful tool for diagnosing and forecasting C fluxes and yield. However, crop model spatial upscaling is often limited by input data (including meteorological drivers and management), parameter uncertainty and model complexity. Earth observation (EO) sensors can provide regular estimates of crop condition over large extents. Therefore, EO data can be used within data assimilation (DA) schemes to parameterise and constrain models. Research presented in this thesis explores the key challenges associated with crop model upscaling. First, fine-scale (20-50 m) EO-derived data, from optical and radar sensors, is assimilated into the Soil-Plant-Atmosphere crop (SPAc) model. Assimilating all EO data enhanced the simulation of daily C exchanges at multiple European crop sites. However, the individually assimilation of radar EO data (as opposed to combined with optical data) resulted in larger improvements in the C fluxes simulation. Second, the impacts of reduced model complexity and driver resolution on crop photosynthesis estimates are investigated. The simplified Aggregated Canopy Model (ACM) – estimating daily photosynthesis using coarse-scale (daily) drivers – was calibrated using the detailed SPAc model, which simulates leaf to canopy processes at half-hourly time-steps. The calibrated ACM photosynthesis had a high agreement with SPAc and local EC estimates. Third, a model-data fusion framework was evaluated for multi-annual and regional-scale estimation of UK wheat yields. Aggregated model yield estimates were negatively biased when compared to official statistics. Coarse-scale (1 km) EO data was also used to constrain the model simulation of canopy development, which was successful in reducing the biases in the yield estimates. And fourth, EO spatial and temporal resolution requirements for crop growth monitoring at UK field-scales was investigated. Errors due to spatial resolution are quantified by sampling aggregated fine scale EO data on a per-field basis; whereas temporal resolution error analysis involved re-sampling model estimates to mimic the observational frequencies of current EO sensors and likely cloud cover. A minimum EO spatial resolution of around 165 m is required to resolve the field-scale detail. Monitoring crop growth using EO sensors with a 26-day temporal resolution results in a mean error of 5%; however, accounting for likely cloud cover increases this error to 63%.