Dissertationen zum Thema „Plant-soil relationships“

Thesis (Ph.D.)--Michigan State University. Dept. of Forestry Program in Ecology, Evolutionary Biology and Behavior, 2008.
Title from PDF t.p. (viewed on July 13, 2009) Includes bibliographical references (p. 170-184). Also issued in print.

The relationship between plants and rhizosphere bacteria collected from a 45 year old permanent pasture was investigated. Several methods of strain identification within Rhizobium trifolii were evaluated. Separation of bacterial isolates based on differences in intrinsic antibiotic resistance was not appropriate because strains developed hybrid resistance patterns when grown in a common broth. Serological analyses of bacterial antigens using polyclonal antiserum yielded two reliable methods for identifying R. trifolii isolates. Agglutination and immunofluorescence procedures were not useful in distinguishing these strains but immunodiffusion and the enzyme-linked immunosorbent assay (ELISA) were highly suitable. Adaptation of the ELISA allowed isolates to be identified directly from individual root nodules without first subculturing the bacteria. A strain of Bacillus polymyxa isolated from the same pasture was shown to stimulate growth of crested wheatgrass (Agropyron cristatum L.) and white clover (Trifolium repens L.). The primary manifestation of the effect was increased root weight (P < 0.05), but shoot responses were also observed. Perennial ryegrass (Lolium perenne L.) generally reacted negatively to inoculation with this bacterium. Further stimulation of growth was noted when ramets of the white clover genotype homologous to (sharing a common origin) B. polymyxa were inoculated in pure stands (P < 0.05). Clones of the homologous perennial ryegrass genotype also showed a yield increase from slightly below control levels to slightly above them when tested in a similar manner. Detailed analysis of the crested wheatgrass response to inoculation revealed that bacterial production of indole acetic acid was the most likely cause of the growth stimulation. Other bacterial characteristics such as the ability to fix atmospheric nitrogen or to solubilize organic phosphorus were concluded to be unrelated to the growth response. Co-adaptive compatibility between genotypes of L. perenne and T. repens was not apparent when the effect of R. trifolii was ignored. However, when clones of pasture plants that had been neighbours in the field were inoculated with R. trifolii isolated from root nodules of the "parental" clover genotype, biotic specialization between the pasture plants became evident. The magnitude of the effect, which was characterized by superior white clover yields (P < 0.05), could be largely accounted for by the presence of the adapted L. perenne/R. trifolii combinations, regardless of the white clover genotype. Since T. repens was the dominant component in the species mixture, these trends were also apparent when total forage biomass was analyzed (P < 0.05). However, ecological combining ability was found to be lowest in these associations (P < 0.05). Similar experimentation with isolates of B. polymyxa (or B. polymyxa-like organisms) was performed. Again the grass/bacteria combination was shown to be influential in the growth response as the presence of homologous L. perenne/B. polymyxa combinations resulted in superior white clover and perennial ryegrass performance (P < 0.05). When T. repens was inoculated with a mixture of R. trifolii strains, unrelated isolates formed more root nodules than did homologous ones (P < 0.05). The presence of perennial ryegrass did not mitigate this effect. However, when homologous R. trifolii was administered as a single strain inoculum, yield advantages in white clover were observed (P < 0.05). If B. pol ymyxa was present, homologous strains of R. trifolii tended to form most of the root nodules regardless of the T. repens or L. perenne genotypes. The significance of the yield advantages observed in various two and three-way plant/microbe genotype combinations is discussed with respect to above ground plant performance.
Land and Food Systems, Faculty of
Graduate

In a field study of irrigated chile (Capsicum annum L.) production in southeastern Arizona and southwestern New Mexico from 2008 through 2009, soil and tissue test samples were analyzed for a full spectrum of nutrients at 16 different sites, including nitrogen (N) phosphorus (P), potassium (K), zinc (Zn), iron (Fe), and boron (B), with the objective of evaluating soil and tissue nutrient testing procedures and establishing basic testing guidelines and recommendations with respect to yield potentials. Results for soil and tissue analysis were correlated to yield results. The results provide estimates for baselines which can be tested through subsequent calibration experiments for the development of recommendations for critical soil and tissue test values. These soil test and plant nutrients values will be evaluated in subsequent experiments in an effort to better define fertilizer nutrient inputs in order to gain better nutrient management efficiencies in irrigated chile production systems.

Thesis (Ph. D.)--University of Missouri-Columbia, 2007.
The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file. Title from title screen of research.pdf file (viewed on September 25, 2007) Vita. Includes bibliographical references.

Growth, productivity and fruit quality of grapevines are closely linked to soil water availability. Withholding of water for any length of time results in slowed growth. If drought continues yield may be lost. Vines can be manipulated to stimulate early defence mechanisms by decreasing soil water availability. By using an irrigation technique, which allows for separate zones with different soil moisture status, it is possible to stimulate response mechanisms of the root system which are normally related to water stress. The difficulty of separating 'wet' and 'dry' zones was initially overcome by using split-root plants with root systems divided between two containers. Such experiments on split-root model plants resulted in the development of an irrigation technique termed partial rootzone drying (PRD). Results from irrigation experiments using PRD have shown that changes in stomatal conductance and shoot growth are some of the major components affected (Dry et al., 1996). The idea of using irrigation as a tool to manipulate stress responses in this way had its origin in the concept that root- derived abscisic acid (ABA) was important in determining stomatal conductance (Loveys, 1984). Later experiments on split-root plants have demonstrated that many effects of water stress can be explained in terms of transport of chemical signals from roots to shoots without changes in plant water status (Gowing et al., 1990). The necessary chemical signals are provided by the dry roots, and the wet roots prevent the development of deleterious water deficits. The general hypothesis tested during this study was that partial drying of the root system gives rise to a change in the supply of root-derived chemical signals which determine changes in grapevine physiology, thereby affecting fruit quality. Experiments were conducted on split-root vines (Vitis vinifera L. cvs. Cabernet Sauvignon and Chardonnay) grown in pots of different sizes, on field-grown vines which had either their root system divided by a plastic membrane (Vitis vinifera L. cv. Cabernet Sauvignon on own roots or grafted on Ramsey rootstocks) or conventional vines with a non-divided root system (Vitis vinifera L. cv. Cabernet Sauvignon, Shiraz and Riesling) with a commercial PRD irrigation design. The irrigation treatments were vines receiving water on both sides (control) and PRD-treated vines, which only received water on one side at any time. The frequency of alternation of 'wet' and 'dry' sides was determined according to soil moisture and other influences such as rainfall and temperature. In most of the experiments the irrigation was alternated from one side to the other every 10 to 15 days. Chemical signals from roots: the role of ABA and cytokinins Studies on chemical signals have concentrated on ABA and cytokinins (CK). An improved stable isotope dilution protocol, which enables analysis of ABA and CK from the same tissue sample, was developed. Analysis of cytokinins focused on zeatin (Z), zeatin riboside (ZR), zeatin glucoside (ZG) and iso pentenyl adenine (iP). Roots are relatively inaccessible, particularly in field situations. To enable easier access to roots of field-grown vines, split-root vines were planted in a trench which was refilled with a sandy soil. This created a homogenous soil substrate and did not restrict root growth while still allowing access to roots under field conditions. Analyses of root samples of field-grown vines have shown that cytokinins and ABA may originate in roots and their concentrations can be substantially altered during an irrigation cycle. Alternating soil water conditions showed that [ABA] in roots on the 'dry' side was significantly higher compared with the 'wet' side. Due to a reduction in CK on the 'dry' side of PRD-treated vines, the ratio between ABA and CK was substantially changed during an irrigation cycle. The ABA levels in root tissue and in petiole xylem sap were negatively related to stomatal conductance. This further suggests that ABA, mostly synthesized on the 'dry' side of the root system, might be responsible for a decline in stomatal conductance. Furthermore, a higher pH of petiole xylem sap was observed in PRD-treated vines which may also contribute to the regulation of stomatal conductance. Studies on stomatal patchiness showed that non-uniform stomatal aperture occurred in field-grown vines under natural environmental conditions and was more abundant under PRD conditions. The degree of stomatal opening, determined by using a water infiltration technique, correlated with measurement of stomatal conductance. Exogenous application of a synthetic cytokinin (benzyl adenine) can override the possible ABA-mediated stomatal closure resulting from PRD treatment, providing further evidence for the in vivo role of these growth regulators in the control of stomatal conductance. The effect of benzyl adenine was transient, however, requiring repeated applications to sustain the reversal. In addition, CKs may also be important in influencing grapevine growth. Following several weeks of repeated spray applications with benzyl adenine, it was found that the development of lateral shoots in PRD-treated vines was enhanced compared to PRD-treated vines sprayed with water only. This supports the idea that the reduction in lateral shoot development seen in PRD-treated vines is due to a reduced production of CKs (Dry et al., 2000a). By measuring shoot growth rate it was found that one common feature of PRD-treated vines, which were not sprayed with CK, was a reduction of lateral shoot growth. It can therefore be speculated that the reduction in lateral growth is related to a reduced delivery of cytokinins from the roots. Zeatin and zeatin riboside concentration in shoot tips and prompt buds/young lateral shoots were reduced by the PRD treatment providing further evidence in support of this hypothesis. Water movement from 'wet' to 'dry' roots Roots, being a primary sensor of soil drying, play an important role in long- and short-term responses to PRD. Using stable isotopes of water and heat-pulse sap flow sensors water movement was traced from wet to dry roots in response to PRD. The redistribution of water from roots grown in a soil of high water potential to roots growing in a soil of low water potential may be of significance with regard to the movement of chemical signals and the control of water balance of roots. Measurements of the relative water content (RWC) have shown a slower decline of RWC of the 'dry' roots of PRD vines relative to roots of vines which received no water, despite similar water content in soil surrounding those roots. The redistribution of water may help to sustain the response to PRD for longer periods possibly releasing chemical signals and to support the activity of fine roots in drying soil. Field vines, irrigated with PRD over several growing seasons, altered their root distribution relative to the control vines. PRD caused a greater concentration of fine roots to grow in deeper soil layers and this may contribute to a better water stress avoidance. The effect on root growth may be augmented by the water movement and by the large difference in ABA to cytokinin ratio, which are also known to alter root growth. PRD makes more efficient use of available water In experiments where both control and PRD-treated vines received the same amount of water many differences between the vines were demonstrated. Under conditions where water supply was adequate for both treatments, the stomatal conductance and growth of the PRD-treated vines was restricted as has been observed in many previous experiments. As total water input was reduced, however, the stomatal conductance of PRD-treated vines became greater than control vines, suggesting that the latter were experiencing a degree of water stress, whereas the PRD-treated vines were not. This may have been due to the greater depth of water penetration in the case of the PRD-treated vines, where water was applied to a smaller soil surface area. This distinction between PRD-treated and control vines, at very low water application rates, was also reflected in pruning weights and crop yields which were actually greater in PRD-treated vines. It was concluded that at low water application rates, the PRD-treated vines were more tolerant of water stress and made more efficient use of available water. Reduction in vigor opens the canopy. The initial aim of the research which led to the development of PRD was to achieve better control of undesirable, excessive shoot and foliage growth which, from a viticultural point of view, has many disadvantages. Grapevine shoot growth rate responds very sensitively to drying soil conditions. The irrigation strategy used in the PRD experiments maintained a reduction of both main shoot and lateral shoot growth. In response to PRD a decrease in shoot growth rate and leaf area was observed. Much of the reduction in canopy biomass was due to a reduced leaf area associated with lateral shoots, thus influencing the canopy structure. This was one major factor improving the light penetration inside the canopy. Control of vegetative vigour results in a better exposure of the bunch zone to light and, as a consequence, in improved grape quality. It is likely that changes in canopy density, as a result of PRD, is causing changes in fruit quality components. Anthocyanin pigments such as derivatives of delphinidin, cyanidin, petunidin and peonidin were more abundant in berries from PRD vines; by comparison the concentration of the major anthocyanin, malvidin, was reduced. When leaves were deliberately removed from more vigorous control vines, which improved bunch exposure, the differences in fruit composition were much reduced. This further supports the idea that a more open canopy, in response to PRD, improves fruit quality by affecting the canopy structure. Fruit quality consequently determines the quality, style and value of the finished wine. Wines from this study have been produced and data on wine quality from commercial wineries are also available. Sensory evaluations have demonstrated that high wine quality from PRD-treated vineyards can be achieved without any yield-depressing effects. This study has provided evidence to support the original hypothesis. The major findings were: a) Chemical signals, altered under PRD and mostly originating from roots, play an important role in the root to shoot communication in grapevines. b) The movement of water from 'wet' to 'dry' soil layers may help to sustain chemical signals as a response of grapevines to PRD and to support the activity of fine roots in drying soil. c) A reduction in vegetative growth, in particular of lateral shoots, was sustained using PRD and affected the canopy structure which in turn, due to a better light penetration into the canopy, improved the fruit quality. d) The reduction in irrigation water applied did not have a detrimental effect on grape yield and thus the efficiency of water use was improved. e) Application of relatively low irrigation rates showed that PRD-treated vines were more tolerant of water stress and made more efficient use of available water.
Thesis (Ph.D.)--Department of Horticulture, Viticulture and Oenology, 2000.

[Truncated abstract] Root-induced changes to soil hydraulic properties (SHP) are an essential component in understanding the hydrology of an ecosystem, and the resilience of these to climate change. However, at present our capacity to predict how roots will modify SHP and the consequences of this is limited because our knowledge of the processes and effects are highly fragmented. Also, current models used to investigate the relationship between plants and root-induced changes to SHP are based on empirical relationships which have limited applicability to the various and often contrasting ecosystems that occur. This thesis focuses specifically on the quantifying the processes by which roots modify SHP and developing models that can predict changes to these and the water balance. Both increase and decreases in saturated hydraulic conductivity have been attributed to the presence of roots. In general, decreases occur when the root system is relatively young, and increases occur when the roots senesce and begin to decay, creating voids for water flow. The evidence available suggests that the change in pore geometry created by roots is the dominant process by which roots modify SHP because they are more permanent and of a greater magnitude than changes to fluid properties or soil structure. We first quantified the effects of wheat roots on SHP of a coarse sand with a laboratory experiment where we measured changes in both SHP and the root system at 3, 5, 7 and 9 weeks after sowing (weeks). ... The main message that can be drawn from this thesis is that root-induced changes to SHP are dynamic, and dependent upon the combination of soil texture, connectivity of root-modified pores and the ratio of root radius to pore radius. Consequently, root-induced changes to the water balance have the same dependencies. The work in this thesis provides a significant first step towards improving our capacity to predict how roots modify soil hydraulic properties. By defining the range for the parameters used to predict how the soil is modified by roots, we are able to make quantitative assessments of how a property such as hydraulic conductivity will change for a realistic circumstance. Also , for the first time we have measured changes in soil hydraulic properties and roots and have been able to establish why a rapid change from a root-induced decrease to increase in Ks occurred. The link between physiological stage of the root system, and the changes that are likely to occur has implications for understanding how roots modify SHP: it may provide an effective tool for predicting when the switch from a decrease to increase occurs. Further work is required to test the validity of the assumptions we have made in our models that predict changes to SHP. While we have endeavoured to define the parameter space for those parameters that we have introduced, there is still some uncertainty about the connectivity of root-modified pores. Also, the parameterisation of the soil domain with roots is based upon work that measures 'fine' roots only which may not provide a true representation of the effect trees and perennial shrubs have on SHP. It is inevitable that root-induced changes to SHP will affect the fate of solutes in the soil, and temporal dynamics of root-induced changes to these may be particularly important for the timing of nutrient and pesticide leaching.

Biological soil inoculants are products that contain beneficial microorganisms with the ability to increase plant-available nutrients. Soil inoculants are claimed to serve as a supplement for fertilizer N, improve germination and tilth, improve root systems, and increase crop quality and yield. The inoculant evaluated in this study was Effective Microorganisms (EM) produced by Nature Farm Foundation of Lompoc, California. A field trial with corn (Zea mays, L.) was conducted at Oakwood Organic Farm in East Central Indiana in 1992. Two soil types were used in the study, a Genesee silt loam (Fluventic Eutorchrept) and a Sloan silt loam (Fluventic Haplaquoll). Soil nitrate level measured through the growing season was not significantly affected by the addition of EM. Neither foliar nutrient content nor grain dry matter yield were significantly greater with use of EM compared to the control.
Department of Natural Resources

Title from PDF of title page (University of Missouri--Columbia, viewed on February 24, 2010). The entire thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file; a non-technical public abstract appears in the public.pdf file. Dissertation advisor: Dr. Dale Blevins. Vita. Includes bibliographical references.

The understanding of the spatial distribution of lead (Pb) in soil is important in the assessment of potential risks and development of remediation strategies for Pb contaminated land. In situ heterogeneity of Pb was measured at two heavily contaminated sites in the United Kingdom using the Portable X-ray Fluorescence Spectrometer (P-XRF) over a range of spatial scales (0.02 to 50 m). The pattern of the distribution of Pb was very variable, and when expressed as heterogeneity factor (HF), it ranged from 1.2 to 3.2 (highly heterogeneous). The effect of such Pb heterogeneity on plant uptake was investigated in greenhouse pot trials. Two earlier pot trials, which assessed the effect of Pb in a fixed concentration (1000 mg/kg) and in a range of concentration (100 to 10000 mg/kg) found a significant effect of the Pb added treatments, when compared to a control treatment (0 mg/kg Pb added). Biomass and uptake varied by 20 to 100% within and between 16 species/varieties. Results enhanced the selection of two species (Brassica napus and Brassica juncea) for further pot trials. A third pot experiment with Brassica napus and Brassica juncea in simplistic binary model of heterogeneity found 20 to 60% lower uptake in the binary treatment, than homogeneous the treatment. Biomass was higher by 10 to 50% in Brassica juncea and 20 to 40% lower for B. napus in the bianary treatment, when compared to the homogeneous and control treatments. The effect of a more realistic in situ heterogeneity on plant uptake was investigated in a further pot trial, which simulated low (LH), medium) (MH) and high (HH) heterogeneity treatments, compared to a homogeneous (HO) treatment. It detected a significant (P < 0.05) impact of heterogeneity on biomass and uptake between treatments and species. Four to five fold lower biomass were recorded in HH treatment, when compared to the HO treatment. Shoot and root uptake in (mg/kg) concentration increased with increasing heterogeneity with peak uptake (twice as high as HO treatment) in LH for B. napus and in HH and MH treatments for B. juncea respectively. Shoot and root Pb masses in (μg) were maximum in HO and MH treatments respectively with 50 to 70% lower Pb mass in the HH treatment. Results showed that response to heterogeneity is species specific. A sub-experiment explored the behaviour of plant roots in HH treatment and found 20 to 80% variation in root biomass between concentric patches with same nominal soil Pb concentrations. This provided insights into varied responses of these species to realistic Pb heterogeneity. The research demonstrated that the presence and extent of in situ heterogeneity of Pb in soil plays an important role in Pb uptake by plants. It also showed that the homogeneous and simplistic binary model of heterogeneity do not give reliable estimates of plant growth and Pb uptake in realistic field conditions. This work has implications for improving the efficiency of phytoremediation of Pb contaminated land, phytomining, reliability of risk assessment and models of human exposure to Pb.

Thesis (PhD)--Macquarie University, Division of Environmental and Life Sciences, Dept. of Biological Sciences, 2005.
Bibliography: p. 178-203.
Introduction -- Study sites -- Leaf characteristics and resource availability -- Insect herbivory and resource availability -- Insect communities and resource availability -- Influence of resource availability on recovery from herbivory -- Conclusions.
This project used the resource availability hypothesis (Coley et al., 1985) as a framework for investigating the relationship between resource availability (as defined by soil nutrients), leaf traits, insect herbivore damage and insect community structure. According to the hypothesis, plants from low resource environments should be better-defended, have longer leaf lifespans and slower growth rates than plants from higher resource environments. Higher resource plant species are expected to suffer higher levels of herbivory and recover faster from herbivory than low resource plant species (Coley et al. 1985). A corollary to this hypothesis is that plants from higher resource sites should support greater densities of insect herbivores than low resource species. Comparisons between high and low resource sites were made in terms of: (i) leaf traits of mature and immature leaves; (ii) phenology of leaf maturation; (iii) herbivore damage in the field and laboratory; (iv) diversity and abundance of herbivorous insect fauna; and (v) ability to recover from herbivory.
Mode of access: World Wide Web.
243 p. ill., maps

Heterogeneity of contaminants in soils can vary spatially over a range of scales, causing uncertainty in environmental measurements of contaminant concentrations. Sampling designs may aim to reduce the impact of on-site heterogeneity, by using composite sampling, increased sample mass and off-site homogenisation, yet they could overlook the small scale heterogeneity that can have significant implications for plant uptake of contaminants. Moreover, composite sampling and homogenisation may not be relevant to target receptor behaviour, e.g. plants, and studies, using simplistic models of heterogeneity have shown that it can significantly impact plant uptake of contaminants. The alternative approach, to accept and quantify heterogeneity, requires further exploration as contaminant heterogeneity is inevitable within soils and its quantification should enable improved reliability in risk assessment and understanding variability in plant contaminant uptake. This thesis reports the development of a new sampling design, to characterise and quantify contaminant heterogeneity at scales, from 0.02m to 20m, using in situ measurement techniques, and 0.005m to 0.0005m, using ex situ techniques. The design was implemented at two contaminated land sites, with contrasting heterogeneity based upon historic anthropogenic activity and showed heterogeneity varying between contaminants and at different spatial scales, for Pb, Cu and Zn. Secondly, this research demonstrates how contaminant heterogeneity measured in situ can be recreated in a pot experiment, at a scale specific to the plant under study. Results, from 4 different plant species, demonstrated that existing simplistic models of heterogeneity are an inadequate proxy for plant performance and contaminant uptake under field conditions, and significant differences were found in plant contaminant concentrations between simplistic models and those based upon actual site measurements of heterogeneity. Implications of heterogeneity on plant roots were explored in the final experiment showing significant differences in root biomass between patches of differing contaminant concentrations.

Thesis (M.S.)--University of Toledo, 2009.
Typescript. "Submitted as partial fulfillment of the requirements for The Master of Science Degree in Biology (Ecology-track)." "A thesis entitled"--at head of title. Bibliography: leaves 44-55.

Plant roots are central to C- and N-cycling in soil. However, (i) plants differ strongly in tissue recalcitrance (e.g. lignin content) affecting their mineralization in soil, and (ii) rhizodeposits also vary strongly in terms of the metabolites that they contain. Therefore, (i) we used 13C labelled ryegrass root and shoot residues as substrates to investigate the impact of tissue recalcitrance on soil processes through controlled incubation of soil, (ii) we assessed variations in root C-deposition between barley genotypes and their respective impacts on soil processes using 13CO2 labelled plants, (iii) using 13C/15N enriched ryegrass root residues as tracer material, we investigated the impacts of barley genotypes on mineralization of recently incorporated plant residues in soil and plant uptake of the residue-derived N, and (iv) we applied a quantitative trait loci analysis approach to identify barley chromosome regions affecting soil microbial biomass and other soil and root related traits. In the first study, addition of root residues resulted in reduced C-mineralization rates, soil microbial activity and soil organic matter (SOM) priming relative to shoot residues. Planted experiments revealed (i) genotype effects on plant-, SOM- and residuederived surface soil CO2-C efflux and showed that incorporation of plant derived-C to the silt-and-clay soil fraction varied between genotypes, indicating relative stabilization of root derived-C as a result of barley genotype, (ii) that plant uptake of residue released N between genotypes was linked to genotype impacts on residue mineralization, and (iii) barley chromosome regions that influence plant-derived microbial biomass C. These results (i) suggest that greater plant tissue recalcitrance can lower soil C-emissions and increase C-storage in soil, and (ii) demonstrate the barley genetic influence on soil microbial communities and C- and N-cycling, which could be useful in crop breeding to improve soil microbial interactions, and thus promote sustainable crop production systems.

This thesis examines the growth response and inter-relationships between shoots and roots of plants grown in compact soil. In the field, two topsoil and two subsoil conditions were created with five vegetable crops sequentially grown. Between 6 and 12% of the root system grew in the compact subsoil, which had a soil strength of 3.1 c.f. 1.9 MPa in the loosened subsoil. Both the root length density (Lv) and the specific root length were lower in the compact subsoil (80% and 30%, respectively). This had no effect on shoot growth when water and nutrients were well supplied. Compensatory root growth in the lose soil above the compact subsoil occurred in broccoli plants. As a result plants grown in soil with or without a compact subsoil had a similar total root length but with altered root distribution. When the water and nitrogen supplied to the soil was reduced, the lower subsoil Lv in the compact subsoil did not restrict water or N acquisition. This was possibly due to a large increase in the specific uptake per unit length of root, by the fewer roots in the compact subsoil. Compared to the subsoil treatments, only small changes in topsoil physical properties occurred when tillage was ceased. From the field trials the proportion and time of root growth into compact soil appeared important in determining the plant response. In a series of split-root experiments (horizontal and vertical arrangements of compact and loose soil) compensatory root growth in the loose soil only occurred when the root system was exposed to horizontally compact soil When compensatory root growth did not occur shoot growth was reduced. This resulted in there being a close relationship between total root length and leaf area. Further test results support a direct effect of mechanical impedance on shoot growth with a rapid (within 10 minutes) and large (50%) reduction in leaf elongation occurring when roots were mechanically impeded. In the field only plants whose roots were totally exposed to compact soil had reduced shoot growth with very compact subsoil having no effect.

The grasslands of British Columbia, Canada are an asset to the province’s biodiversity, economy, natural beauty, and recreation. Since the late 1800s, these areas have been largely modified and reduced, and they are currently threatened by climate change and exotic plant invasions. The ability of land managers to adapt to the effects of climate change and plant invasion depends on having the best possible understanding of these systems; however, little quantitative information is available on the strength of relationships between topography, soil properties, and plant community composition. I collected data on plant communities, selected soil properties, and topography on 31 sites in Lac du Bois Provincial Grasslands Park in southern interior British Columbia during May-July 2010. Cluster analysis (UPGMA) and multi-response permutation procedures (MRPP) of my data produced more homogeneous groups of sites by vegetation (A=0.29, p=0.001) and soil properties (A=0.22, p=0.001) than for two currently used grassland classification systems that are based on elevation ranges (A=0.17, p=0.001; A=0.15, p=0.001). Non-metric multidimensional scaling (NMS) confirmed the distinctiveness in overall species composition and main environmental correlates for the three main plant communities: bluebunch wheatgrass/big sage, bluebunch wheatgrass/rough fescue, and rough fescue/Kentucky bluegrass. The Mantel (multivariate) correlation between plant community composition and soil properties was stronger on north-facing sites (0.58) than on south-facing sites (0.19), while the Mantel correlation between topography and soil properties was stronger on south-facing sites (0.38) than on north-facing sites (n.s.). These correlations indicate stronger plant-soil feedbacks on north-facing slopes, while on south-facing slopes the effect of slope angle on heat and desiccation stress is the most important factor shaping plant communities. The highest occurrences of invasive species in the study area were at the higher elevations, likely in response to increasing precipitation, lower temperatures, and higher soil fertility at the higher elevations. The information provided in this thesis can aid land managers by adding to current knowledge about the relationships between plant communities, soil properties, and topography, which is important for predicting effects of future climate change and the spread of invasive plant species.

Thesis (MScAgric (Soil Science))--University of Stellenbosch, 2010.
ENGLISH ABSTRACT: The Renosterveld of the Western Cape region is often seen as a natural occurring veld type that will very easily re-establish itself wherever land is left unattended. In this study it was firstly noted that where wheatlands of the Berg River catchment (BRC) is left bare for a number of years, the renosterbos as a pioneer is slow in its re-growth response and when it does, certain patches in the landscape are preferred. This study therefore firstly focussed on the soil restrictions that widely determined the positions in the Berg River landscape where the renosterbos will re-establish itself. Secondly we needed to know whether some of the soil restrictions encountered could be alleviated and was possibly due to cultivation of this land. Through aerial observation it was found that a general patchiness does exist in the naturally occurring Renosterveld of the Voëlvlei area and hill tops of the region and was described by others as the true nature of this veld type. Closer investigation of the soils in the Voëlvlei reserve however showed that soil type played a major role in the patchiness found here. When re-growth of the renosterbos in previously cultivated areas was investigated, it was found that the soil type played the major role in the patchiness that occurred. The most commonly found soil restriction was soil density of the lower horizons. Any soil form that prevented the renosterbos to access the perched water table, to about 15m depth could not support the renosterbos. It is however our belief that soil could be prepared for the re-growth of renosterbos and through this action; renosterbos could also be used to alleviate the salinity problems found in this region. Additionally we investigated the impact of land-use change on the soil water balance and soil salinity by comparing a mature re-established stand of Renosterveld with an adjacent wheatfield. From the results, large differences in salinity and soil water behaviour were detected between the Renosterveld and wheatfield. Modelling of soil and plant water relations was done and the results were correlated well with field observations. This research also confirmed that the renosterbos through its deep rootedness is crucial in the conservation of other species found in the Renosterveld resulting from its ability to keep the water table down and with that the salts that is so often a problem in this area.
AFRIKAANSE OPSOMMING: In die Wes-Kaap word Renosterveld gesien as 'n veld tipe wat natuurlik voorkom en maklik sal hervestig in areas waar land sonder toesig gelaat word. In hierdie studie is dit eerstens opgemerk dat waar koringlande in die Berg Rivier opvanggebied kaal gelaat word vir 'n aantal jare, is die renosterbos as pionier stadig in sy hervestiging en wanneer terug groei wel plaasvind is dit selektief. Die studie fokus dus eerstens op grondbeperkinge wat die areas bepaal waar Renosterveld sal hervestig. Tweedens wou ons vasstel of die grondbeperkings wat voorkom in die grond en wat heel moontlik die oorsaak is van landbewerking opgehef kan word. Deur lugfoto-waarneming is dit gevind dat algemene leë kolle wel opgemerk is in die natuurlik plantegroei van die Renosterveld, in die Voëlvlei area, asook teen die berg hange. Dit word beskryf as 'n algemene kenmerk van die Renosterveld. Nadere ondersoek in die verskillende grondtipes van die area het egter gewys dat die grond tipe 'n belangrike rol speel in die voorkoms en groei van die renosterbos en uiteindelik die (her-)vestiging van Renosterveld. Die terug groei van die renosterbos is ondersoek in voorheen bewerkte lande. Dit is gevind dat die grond tipe 'n belangrike rol speel in die voorkoms van die leë kolle in die Renosterveld. Die mees algemene grond beperking wat opgemerk is, was die verdigte sub-horisonte. Enige grondvorm wat toegang van die renosterboswortels tot by die grondwatertafel (tot by 'n diepte van 15m) beperk, is nie voldoende om die groei van 'n volwasse renosterbos te onderhou nie. Dit is egter ons oortuiging dat die grond voorberei kan word vir die hervestinging van die renosterbos en deur dit te bewerkstellig sal grondversouting beheer kan word. Die impak van landgebruikverandering op die grondwaterbalans en grondversouting is ook ondersoek, deur 'n volwasse stand van Renosterveld te vergelyk met 'n nabygeleë koringveld. Die resultate het getoon dat daar groot verskille in die grondwatervlakke, asook die soutinhoud tussen die Renosterveld en die koringland voorkom. Modellering van die grond-en plantwaterverhouding is uitgevoer en data het goed gekorreleer met veld waarnemings. Die studie het bevestig dat die natuurlike bewaring van die diep gewortelde renosterbos noodsaaklik is vir die voortbestaan van blom- en skilpadspesies wat slegs in die Renosterveld voorkom asook die vermoë van die renosterbos om stygende watertafels en versouting te beheer waar dit dikwels 'n probleem in hierdie area is.

The Ethiopian Highlands have been studied extensively, hosting a large amount of research for development projects in agriculture and forestry over several decades. The encounters in these projects were also encounters of different ways of knowing that were negotiated by the actors meeting in the space provided by the projects. This research explores these encounters and the social worlds they are embedded in, drawing on actor-oriented approaches as well as theories of narratives and framing. Ways of knowing and citizen epistemologies are taken as a lens to understand the role of identities in knowledge production and use. The two case studies were agroforestry research projects in the Ethiopian Highlands. The research followed a range of qualitative and ethnographic research methods. Different types of farmers and scientists meet in the case studies. I recognise that they all have individual agency, nevertheless I use the terms ‘scientist' and ‘farmer' in this thesis. I use the terms to describe certain groups of actors who all draw on different ways of knowing, and different value systems, when interacting with each other and their environment. The results indicate that the importance of social worlds at different scales and the contexts of research projects tend to be underestimated. In spite of good intentions scientific methodologies, terminologies and narratives tend to dominate. Scientists in the case studies acknowledged the existence of farmers' ‘indigenous' knowledge, but they determined the value of knowledge by its scientific applicability and the replicability of experiments. Research systems force the scientists into a certain modus operandi with limited possibilities to experiment and to respond to the complexities and diversities of people's social worlds. Farmers in the case studies preferred observation from their parents, observing from others or the environment as a way of learning and gaining knowledge. Depending on their personalities and their life histories they also relied on alternative ways of knowing rooted in spirituality, emotions and memories. Powerful influences on ways of knowing resulted from the way languages and authority had been used. These often led to the exclusion of marginalised community members from access to knowledge and technologies. Unfortunately, common narratives prevailed in the case studies, and alternative ways of knowing were often marginalised. By acknowledging different ways of knowing and the importance of different social worlds and different ways of doing research, both scientists and farmers could benefit and develop more sustainable pathways for agricultural and forestry land use.

Mechanisms of root growth under variable field conditions were investigated by observing corn ( Zea mays L.) root growth and distribution in the field and by observing the influence of soil physical stresses on corn seedling root growth in controlled environments. The field soil was Groseclose silt loam (clayey, mixed, mesic Typic Hapludult). Groseclose A horizon material was used for the growth chamber experiments where corn was grown in a range of aggregate sizes, bulk densities, low and high soil moisture levels, and temperatures. Rooting patterns in the field were altered by drought. Root length density decreased in the dry surface soil and proliferated in the moist subsurface soil. Distribution of roots length densities was skewed. A few samples contained many roots and many samples contained few roots because roots were restricted to interpedal voids. In the growth chamber experiments, roots were not able to penetrate large aggregates and were restricted to interaggregate zones. This tortuous path of root growth led to transitory impedances as roots were deflected around aggregates. Corn roots were able to push small aggregates out of their path. An equation was developed to describe this impedance as a function of aggregate size, root diameter, and deflection angle. Mechanical impedance, oxygen stress, lower temperatures, and moisture stress reduced seedling root elongation to some extent, but the influence of reduced temperature was the most dramatic. At 6 days corn root length at 21°C was 20% of that at 25°C while root length at 17°C was only 5% of that at 25°C. Mechanical impedance and reduced temperatures also increased root diameter. In wet soil, oxygen stress was the most immediate factor affecting root growth, but after 4 days root elongation was stimulated suggesting other unknown factors. Two semi-empirical models were developed. One was based on the exponential growth rate of the root system and the other based on the linear growth rate of each root member. These models accounted for the reduction in root growth rate due to the soil physical stresses.
Ph. D.

Thesis (M.S.)--University of Toledo, 2009.
Typescript. "Submitted as partial fulfillment of the requirements for The Master of Science Degree in Biology (Ecology-track)." "A thesis entitled"--at head of title. Bibliography: leaves 28-32.

In Australia a class of soils known as sodic duplex soils covers approximately 20% of the continent. Their defining characteristic is a sharp texture contrast between the A (or E) and B horizon. The upper B horizon at the point of contact with the E horizon is often highly sodic and of such a high strength that root growth and proliferation, water conductivity, aeration, water storage and water uptake are restricted. Roots growing in these soils rely on channels created by previous roots or cracks arising from shrink– swell forces associated with seasonal wetting and drying. It has been suggested that by increasing the number of these channels in the subsoil, the structure and permeability of the subsoil would be increased as would be the number of preferential pathways for following generation roots. A biological approach for improving soil macroporosity would be to use plants that can grow through that hostile layer creating new channels. This is known as the primer plant concept. This concept is based on a better understanding of root soil interactions. It is accepted that root growth is influenced by the soil structure and the soil structure is influenced by root growth. However, a lot of these dynamics are still unknown. This project aims to contribute to improving that knowledge by investigating the use of modern techniques to study plant/root interactions in duplex soils. First macroporosity and mesoporosity were characterized in three dimensions using medical computer tomography and micro-tomography. Then the imaging methodology was improved by using a local and adaptive threshold technique based on indicator kriging instead of a global threshold. Using this new methodology, changes in porosity were analysed in intact samples when three different plant species were grown for 12 weeks. The plants were canola (Brassica napus); lucerne (Medicago sativum) and saltbush (Atriplex nummularia) hypothesizing saltbush would change the porosity more because it is a native plant species based on the primer plant concept. The results showed that the porosity changed significantly after root growth but no ii differences were found between plant species. The changes could also not all be attributed to root growth because cracks were also formed after 12 weeks. Therefore, the living roots were visualized and characterized using a new tracing algorithm 'rootviz'. This revealed that saltbush was growing more roots down through the profile. Lucerne seemed to grow roots down the profile as well but to a lesser extend. Both of these plants seemed to have more geotropic features than canola that seemed to grow more laterals and had a more exploratory behaviour.

Soils formed from ultramafic rocks are normally by pH values close to neutrality, a high base status and are usually rich in Mg, Fe and heavy metals. The low Ca/Mg ratio and the high heavy metal content could cause toxic effects in the biological communities. Plant communities, in particular, are usually different from nearby areas with different substrates and rich in endemisms and adapted species and subspecies. Despite their great environmental and ecological interest, pedological and ecological properties of mountain or boreal soils developed on similar substrates have seldom been studied worldwide. 198 soil pits (associated with phytosociological surveys) have been opened and analyzed in the ophiolitic area of Mont Avic Natural Park (Val d’Aosta, Western Alps, Italy), beween 900 and 2900 m above see level. Soils formed from ultramafic, mafic rocks and calcschists have been observed, in order to recognize the most ecologically important soil factors. The results show that soil properties are related with altitude and slope aspect in forest habitats, while the effect of substrate becomes important above timberline. Strong leaching in forest soils, related to high acidity and to the podzolization process, decrease the total and bioavailable heavy metal contents, above the treeline pedogenic and geomorphic processes release and accumulate large quantities of potentially hazardous trace elements. The plant communities strictly depend on the edaphic properties above the treeline, while in the forest habitats the differences caused by substrate are less discernible. Microbial and microarthropodal communities suffer stress caused by heavy metals in forest soils, while at the alpine level non significant statistical or ecological correlation are visible. Heavy metals (Ni, in particular) are the most important edaphic properties in differentiating plant communities on different substrata, while the Ca/Mg ratio (usually considered the most influencing soil properties on ultramafic soils) has no particular ecological effect.

The Matsqui area of the Lower Fraser Valley exhibits extreme soil heterogeneity, as the alluvial soils in the area have been deposited by the Fraser River as a series of coarse-textured ridges and finer-textured depressional areas. This variability poses some obvious problems with respect to agricultural management. The main aim of this study was to evaluate soil spatial variability in four fields, and to relate this soil variability to corn production and quality. Site conditions, topography, and soil chemical and physical variables were related to corn biomass and nutrient concentrations using conventional correlation/regression analyses, and more spatially representative techniques such as those provided by remote sensing and geographic information systems. Variations in such biophysical variables as soil moisture, elevation, and bulk density had consistent impacts on corn productivity, although these effects varied from field to field, being influenced by inherent soil properties and individual field management. Good relationships were found between pixel brightness values extracted from digitized colour infra-red photos and corn quality variables. In three out of four fields, near infra-red pixel values gave good estimates of total corn crude protein content. Significant relationships were also found between pixel brightness values and corn phosphorus and calcium contents in certain fields. The spatial variability of corn quality and biomass could be quantified using image analysis classification techniques. The resulting classified images indicate to the farm operator where high vs low quality corn is being produced, and thereby provide a tool for selectively managing and harvesting the fields. The relationships and quantification of corn productivity and quality in the fields can further be improved through incorporation of the image data with the biophysical data base using GIS techniques. A multiple regression equation showing a significant relationship between elevation and pixel brightness values, and total corn phosphorus concentration was incorporated within the GIS to produce a quantitative corn quality map for the field exhibiting this relationship. The GIS overlay capability facilitates the classification of several corn variables, and allows the results to be displayed in a spatial manner. For example, corn biomass and quality maps were overlain using GIS techniques, to produce a combination map which then reflected both the quality and quantity of corn found in the field. Through integration of remote sensing and GIS techniques, soil and crop variability can be displayed in a spatial manner. The output from such procedures can aid farm operators in making selective field management and harvesting decisions.
Land and Food Systems, Faculty of
Graduate

[Truncated abstract] A greater adoption of stubble retention, minimum-till and no-till farming practices for the purposes of conserving soil, water and fertility requires a greater understanding of the complexity of physical and chemical interactions between the soil and crop residues. There is currently insufficient knowledge to allow reliable predictions of the effects of different residue types in different environments on soil fertility and crop growth, owing to the many residue characteristics and environmental interactions that have been shown to affect decomposition or nutrient release. The role of fibre and nutrient composition in nutrient release from crop residues, and implications for residue management techniques, were studied. Canola, lupin and field pea residues, obtained from farmland in Meckering and Northam, Western Australia, were separated into upper and basal stems, leaves, and siliques or pods. This was done to provide materials with a wide range of chemical and physical characteristics, and also allowed consideration of differential residue management of plant organs, such as comparing harvested canola siliques and retained canola stubble. Pre-treatment by chopping and/or humidification was applied to residues to provide some information about the processes of nutrient release. Residues were subjected to simulated rainfall to assess nutrient leaching from plant material, and placed on soil in pots in constant-temperature glasshouse conditions to assess decomposition. Amounts and rates of change of residue fibre and nutrients were determined throughout leaching and decomposition. Energy Dispersive X-ray (EDX) microanalysis was used to assess the location of diffusible ions in air-dried residues and the effects of humidification on nutrient positioning and release. ... However, the release of calcium and magnesium depended on the decomposition of the more recalcitrant components such as cellulose and lignin, as supported by microscopy results showing changes in nutrient distribution following humidification. The proportionality of amounts of calcium and magnesium leached and released during decomposition is likely to suggest a similarity of chemical form more than similarity of function or position of the two elements. Management of crop residues for maximising and optimising the timing of release of different nutrients will need to take into account the placement of different plant types and parts, particle sizes distribution and pre-treatment of material to efficiently manage short- and long-term soil fertility to sustain crops, particularly on degraded soils. Significant nutrient release of potassium, sulphur and magnesium from crop residues can be achieved from surface placement, with the release of potassium and sulphur managed by modifying residue particle size through appropriate harvesting, ploughing or sowing implement selection. High nutrient uptake crops and plant parts –where they can be economically viable to grow or separated by the harvesting technique – are particularly valuable as sources of nutrients and soil organic matter.

Thesis (MScConsEcol (Conservation Ecology and Entomology)--University of Stellenbosch, 2007.
Understanding the drivers and mechanisms of changes in plant richness is a basis for making scientifically sound ecological predictions and land use decisions. Of the numerous factors affecting plant richness, soil has a particularly large influence on the composition and structure of terrestrial flora. Infiltrability is one of the most important factors determining soil moisture, and therefore is of particular interest in semi-arid ecosystems, where water is one of the most limiting resources. Other soil properties, such as clay + silt content, electrical conductivity (EC) and pH may also influence plants. Heterogeneity of these properties creates niches with specific conditions, which in turn affects spatial distribution of plants. An understanding of the relationships between plant richness and soil properties is, however, incomplete. The present study has two main foci. Firstly, relationships between plant richness and soil infiltrability, clay + silt, EC and pH (H2O) were investigated, and secondly, due to the strong influence of infiltrability on plant richness, further investigations were undertaken to improve the understanding of the role of particle size fractions, EC of the soil solution and exchangeable sodium percentage (ESP) on infiltrability. This study only concentrated on the surface 2 cm thick soil layer (known as pedoderm).