<p> In this dissertation, I asked how soil biota, abiotic stress, and plant provenance influence plant communities and interactions between plants. Soil biota can have positive or negative effects on individual plants, and also influence the diversity and productivity of plant communities through their net effects on individuals and by mediating plant-plant interactions. However, the level of abiotic stress experienced by plants is likely to drive plant responses to soil mutualists and antagonists. Additionally, plant provenance (e.g. population origin) can influence responses to abiotic soil conditions as well as to soil organisms. Understanding how these three interacting components shape plant interactions may improve success of restoration and invasive plant management. During restoration, the goal is typically to create conditions conducive to native plant reestablishment. However, amelioration of disturbed areas by reducing abiotic stress or by adding beneficial soil organisms may unintentionally increase colonization and growth of non-native plants. Using the applied context of mine restoration, I examined how soil biota, abiotic stress, and plant provenance affected plant communities and interactions in four studies. </p><p> In Chapter 1, I found that both a native grass (<i>Bouteloua gracilis </i>) and an invasive grass (<i>Bromus tectorum</i>) responded positively to soil biota when grown alone in the greenhouse. However, when grown together, the presence of soil biota increased the competitive ability of <i>Bromus,</i> while the removal of soil biota increased competition by <i>Bouteloua.</i> Results supported the hypothesis that invasive species such as <i>Bromus</i> often have positive responses to soil biota in the invaded range, but I also found that <i>Bromus</i> response to soil biota removal varied considerably by site. </p><p> In Chapters 2 and 3, I examined how methods used during restoration (application of stockpiled soil and inoculation with soil biota) affected native and non-native plant growth in field plots. I found that native plant biomass and non-native plant biomass both tended to increase when soil abiotic stress was ameliorated through the addition of deeper stockpiled soil. In addition, both native and non-native grasses responded positively to the use of local soil an as inoculant, while non-native forbs responded negatively to local soil inoculum. However, native plants only received significant benefits from inoculation when targeted application to native seedling transplants was used. Commercial mycorrhizal fungal inoculum did not affect plant growth. In studies of both stockpiled soil addition and soil inoculation, year was an important factor in determining plant responses. Variation in effects by year may reflect differences in precipitation timing or amount, or changes associated with plant and soil biota growth over time. </p><p> In Chapter 4, I used a greenhouse experiment to examine how one type of soil biota, arbuscular mycorrhizal fungi (AMF), influenced plant-plant interactions. I also manipulated abiotic stress (soil phosphorus availability) and plant provenance (stress-tolerant ecotype versus competitive ecotype) to assess whether these factors influenced AMF-mediated interactions among plants. I found that allowing or denying AMF hyphal access between neighboring pots altered plant reproduction. Inflorescence production was substantially decreased when hyphal access was allowed between two stress-tolerant plants. In addition, when hyphal access was permitted from a stress-tolerant plant to a competitive plant, the competitive plant flowered slightly sooner, whereas allowing hyphal access between two stress-tolerant plants led to slightly slower flowering. These results did not appear to be driven by abiotic stress or plant nutrition. It is possible that AMF transmission of infochemicals may play a role in regulating plant phenology and reproduction; however, further research in this area is needed.</p>

<p> Methylmercury (MeHg) is a toxic and bioaccumulative form of mercury that can be produced by bacteria living in water saturated soils, including those found in flooded rice fields. In the Sacramento Valley, California, rice is grown on 240,000 hectares, and mercury is a concern due to a history of mining in the surrounding mountains. </p><p> Using unfiltered aqueous MeHg data from MeHg monitoring programs in the Sacramento River watershed from 1996 to 2007, the MeHg contribution from rice systems to the Sacramento River, was assessed. AgDrain MeHg concentrations were elevated compared to upstream river water during November through May, but were not significantly different during June through October. June through October AgDrain MeHg loads (concentration &times; flow) contributed 10.7&ndash;14.8% of the total Sacramento River MeHg load. Missing flow data prevented calculation of the percent contribution of AgDrains in November through May. </p><p> Field scale MeHg dynamics were studied in two commercial rice fields in the Sacramento Valley. The Studied fields had soil total mercury concentrations of 25 and 57 ng g^-1, which is near the global background level. Surface water and rice grain MeHg and THg concentrations were low compared to previously studied fields. An analysis of surface water drainage loads indicates that both fields were net MeHg importers during the growing season and net MeHg exporters during the fallow season. </p><p> Since the microbes that produce MeHg prefer flooded environments, management that dries the soil might reduce MeHg production. Conventional continuously flooded (CF) rice field water management was compared to alternate wetting and drying, where irrigation was stopped twice during the growing season, allowing soil to dry to 35% volumetric moisture content, at which point plots were re-flooded (AWD-35). Compared to CF, AWD-35 resulted in a significant reduction of MeHg concentration in soil, surface water and rice grain.</p><p>

Allelopathy, secondary metabolite–mediated plant-to-plant interaction, is gaining application in current agricultural science as well as in invasion ecology. The present study addresses the major knowledge gaps in this field by investigating (i) how the bioavailability of allelochemicals is altered when they are present in a mixture in the soil matrix and (ii) what are the cues and consequences of allelochemical production in nutrient acquisition by the plant. ^ The role of preferential sorption to soil in altering the chemical composition of plant exudates was studied in a silt loam soil using representative mixtures of plant phenolic acids, namely, hydroxybenzoic acid, vanillic acid, coumaric acid, and ferulic acid. The concentration-dependent sorption coefficient ( Kd) of hydroxybenzoic acid was decreased more by than 90% in the presence of coumaric acid. About 95% of sorbed vanillic acid was displaced into the soil solution in the presence of ferulic acid. Soil organic matter was associated with preferential sorption. The results demonstrate that preferential sorption of phenolic acids to soil can alter the availability of plant exudates in mixtures and thus may mediate their phytotoxic effects. ^ To understand the dynamics of allelochemical mixture in soil matrix, using Centaurea maculosa Lam.as a model source, we investigated how the bioavailability of complex allelochemical mixtures is modified in a soil-microbial system. C. maculosa litter decomposition experiment revealed the existence of allelochemical in complex mixture in soil matrix. We observed the prolonged persistence of allelochemicals in soil matrix when present in complex mixtures. Also, allelochemicals exhibited dynamic nature undergoing simultaneous degradation as well as synthesis. ^ We investigated allelopathy as a corollary effect of resource acquisition mechanism using Centaurea diffusa Lam. as a model system. The exudation of 8-hydroxyquinoline (8HQ) by C. diffusa was not correlated with any of the tested nutrient stresses; however the 8HQ production remained steady under prolonged Fe stress, indicating this as a specific plant response to Fe stress. 8HQ showed high specificity in extracting Fe from both invaded and non-invaded soils. C. diffusa was able to uptake Fe from Fe(OH)3, but addition of carbon resulted in Fe deficiency, indicating a direct role of 8HQ in Fe uptake by this plant. Related (C. maculosa, C. solistitialis) as well as non related species (Zea mays ) of C. diffusa were able to uptake Fe from Fe-hydroxyquinoline complex (FeQ). Combining the above with the root reductase activity of the species, we propose that FeQ because of its hydrophobic nature, could passively diffuse across plasma membrane. In bioassay studies the addition of Fe decreases the toxicity of 8HQ, suggesting the phytotoxic action of 8HQ is via chelation of cellular Fe. ^ In conclusion, the present study revealed the competitive sorption phenomenon of plant secondary metabolites in soil matrix, as well as their dynamic nature. Combining the results from these studies that, 8HQ not only mobilizes the nutrients from soil, but also these nutrient complexes can be taken up by plants, we report an additional role of 8HQ in increasing the competitive ability of this weed - by facilitating the resource acquisition. Also, we report for the first time, the possible existence of a phytosiderophore-like mechanism for Fe acquisition in non-graminaceous species. ^

<p> One of the least researched phenomena within the alpine regions of mountain biomes is the combination of primitive plants, algae, fungi, and lichens that are generally referred to as biological soil crusts. Sites containing well-developed biological soil crusts were examined in a variety of alpine, non-forested, vegetated landscapes in the North Cascade Mountains of Washington, USA. For each site, data were recorded for percent ground cover of biological soil crusts, slope aspect, and slope gradient of the terrain where the crust communities were located. For all of the sites, biological soil crusts were common, with a percent ground cover median of 29% and a range of 11% to 73%. The arrangement of the biological soil crusts on all sites was quite similar: all were clumped, as opposed to single, and random, as opposed to uniform. All of the soil crusts were found on soil exposed to direct sunlight. Few, if any, crusts were found in the shade of heavy forbs, or forest, or under accumulations of organic litter. When biological soil crusts were found associated with higher-order vegetation, it was with sparse graminoids, ericaceous woody shrubs, and stunted or krummholz Pinaceae trees. The biological soil crusts from this study exist on all locally undisturbed soil slope-gradients from 0% to almost 100%, and occurred on all aspects except for those in the Southwest quadrant. This study contains an extended literature review for desert and high latitude circumpolar crusts, as well as alpine biological soil crusts. Studies of biological soil crusts in subalpine and alpine environments are not common; it is hoped that this study will stimulate more research interest in these often overlooked pioneer biotic communities.</p>