Root-associated (rhizosphere and rhizoplane) microbial communities influence plant phenotype, growth, and local abundance, yet the factors that structure these microbial communities are still poorly understood. California landscapes contain serpentine soils, which are nutrient-poor and high in heavy metals, and distinct from neighboring soils. Many plants are unable to grow in serpentine soils and some endemic species cannot compete on non-serpentine soils. Serpentine-indifferent plants, however, can do both. I utilize this class of plants and a serpentine ecosystem to disentangle the relative influences of plant species and soil type on rhizosphere microbial community composition. In Chapter 1, I characterized the microbial communities associated with the rhizoplane of serpentine-indifferent plants growing on serpentine at McLaughlin Natural Reserve. I supplemented that survey with a manipulative greenhouse experiment where I amended sterile serpentine soil with serpentine-adapted microorganisms, non-serpentine-adapted microorganisms, or a sterile control solution. I then measured seedling survival and plant growth. The results of this experiment showed that plant identity was more important than soil type for structuring rhizosphere microbial communities. Also, soil microbial community sources influenced seedling survival, but plant growth phenotypes were largely invariant to microbial communities with a few exceptions. The results from this experiment are published in Plant and Soil (Igwe, A.N. & Vannette, R.L. Plant Soil (2019) 441:423). In Chapter 2, I used 16S rRNA sequencing to determine how drought impacted the rhizosphere microbial community of several species of Streptanthus . Several species of Streptanthus were exposed to high, medium, and low watering treatments. Bacterial abundances were not significantly impacted by watering treatment. The bacterial communities of the lowest and highest watering treatment were significantly dissimilar. Results showed that alpha diversity decreased as watering levels decreased. Plant species and soil affinity did not impact alpha diversity. Several genera within Proteobacteria, Firmicutes, Bacteroidetes, Planctomycetes, and Acidobacteria were differentially abundant between watering treatments. Microbial community dissimilarity was impacted by watering treatment and species, but not soil affinity. Watering treatment shifted the microbial communities such that less water created microbial communities that are more similar. Overall, this research serves to provide insight into the microbial communities shifts we could expect as a result of drought. In Chapter 3, I conducted a manipulative greenhouse experiment using Plantago erecta. I extracted DNA from rhizosphere microbial communities of P. erecta plants at distinct developmental stages: seedling, vegetative growth, early flowering, and late flowering. The plants were grown in serpentine or non-serpentine soil types with adapted or non-adapted microbes. Plant height and leaf number was measured weekly until harvesting and the plant developmental stage was noted. Afterwards, dry mass of above ground parts was collected, and roots were imaged using the WinRhizo system.16S rRNA amplicon sequencing and data analysis showed that alpha diversity was significantly lower in serpentine soil treatments and plant developmental stages. The variation observed in the rhizosphere microbial community was influenced by soil type, plant developmental stage, and the interaction between them both. Plants associated with serpentine microorganisms flowered sooner than those associated with non-serpentine microorganisms. In general, plants growing on serpentine soils were shorter, but leaf number was not impacted. Root length, root surface area, and root volume were all larger in nonserpentine soil treatments, but root diameter was not significantly different across soil types. These results are important for understanding how microbial communities shift to support plant survival on stressful soils.

General introduction; Land use impact on the stability of soil microbial community composition and enzyme activities to heat shock; Shifts in microbial community composition and physiological profiles across a gradient of induced soil degradation (GRIND); Development and validation of a soil quality index based on the equilibrium between soil organic matter and biochemical properties in an undisturbed forest ecosystem. The objectives of this thesis were to evaluate the responses of soil microbial communities to physical and chemical disturbances, and associate these responses with soil functional stability and changes in soil quality. The first study consisted of application of heat shocks (HS) to soils with contrasting land use history to evaluate differences in the stability of soil enzymes (laccase, cellulase and fluorescein diacetate hydrolysis) and microbial community composition as determined by phospholipid fatty acid (PLFA) analysis. The conversion of land use from forest to agriculture resulted in a new microbial community that was less functionally stable. Loss of stability was indicated by the reduced of laccase and cellulase activities in the agricultural soil, which suggested a less diverse community of microorganisms capable of producing these enzymes. The second study examined changes in microbial community composition and diversity that occurred across a gradient of soil disturbance. Disturbances were simulated by tillage events applied at different intensities to a 12-year-old fallow area. These treatments caused degradation of several soil physico-chemical properties, and alterations in microbial structure based on PLFA and terminal restriction fragment length polymorphism (T-RFLP) analyses, and in metabolic potential based on community level physiological profiles (CLPPs). Multivariate ordination of soil properties revealed the formation of a linear gradient of soil degradation that was significantly correlated with CLPPs, but not with T-RFLP and PLFA profiles. Nevertheless, changes observed in microbial community structure were significantly associated with decreases in soil organic C and field hydraulic conductivity. The third study demonstrated that undisturbed forest soils from western Oregon express an equilibrium between soil organic matter and biochemical properties. A model fitted through multiple regression analysis showed that phosphatase activity and microbial biomass were able to explain 97% of the soil organic C in these soils. This equilibrium was disrupted when a soil from an old-growth site was submitted to chemical stresses (Cu addition or pH alteration) and physical disturbances (wet-dry or freeze-thaw cycles). The magnitude of this disruption was consistently expressed by the ratio between soil C predicted by the model (Cp), and soil C that was measured (Cm). This ratio is proposed as biochemically-based index of soil quality.

Soil microbial communities were examined in two chronosequences of reclaimed surface mine soil and in a chronosequence of previously burned sites to determine how these communities recover through time after disturbance. Phospholipid fatty acid (PLFA) methods for microbial community analysis were used to quantify microbial biomass, diversity, and abundance of specific microbial groups (gram positive bacteria, gram negative bacteria, fungi, arbuscular mycorrhizal fungi, and actinomycetes). Multivariate analysis of variance (MANOVA) and discriminant analysis (DA) were used to compare soil microbial communities and site environmental factors. The soil microbial community in sagebrush-grassland ecosystems disturbed by fire appeared to recover to similar levels of biomass and diversity as in unburned soil within 3 to 7 years. In the surface mine reclamation sites, microbial recovery was seen in 5 to 10 years after reclamation in sites dominated by sagebrush and within 14 years after reclamation in the sites dominated by cool season grasses. Plant community composition was found to have influences in soil microbial recovery. Microbial communities in soil under crested wheatgrass (Agropyron cristatum) recovered to greater biomass than did the communities associated with other plant species. Soil fungi appeared to be the most adversely affected by soil disturbance associated with surface mining than the other microbial groups examined, and they were also slow to recover after the initiation of reclamation. A general trend of recovery towards the undisturbed condition with reclamation age was found for all microbial groups after disturbance. Our data on microbial community recovery from fire and impacts of surface mining suggests that soil microbial communities are highly resilient to disturbances.

Understanding the response of soil microbial communities and decomposition to global environmental changes is central to our ability to accurately forecast future terrestrial carbon (C) storage and atmospheric CO2 levels. Increases in the frequency and severity of disturbance events are one element of global change in terrestrial ecosystems. The goal of this dissertation was to measure the response of soil microbial communities and decomposition to disturbance events and to examine the mechanisms underlying post-disturbance changes in decomposition. In the first part of my dissertation work I explored these questions within the context of wildfires in boreal forests. Chapter 1 characterized soil microbial communities and the rate of decomposition across a fire chronosequence in interior Alaska. I found that boreal forest fires reduced soil microbial abundance, altered fungal community composition, and suppressed litter decomposition. Chapter 2 investigated whether soil microbial responses to boreal forest fires differ as a function of fire severity. I demonstrated that higher severity fires elicited greater reductions in soil microbial biomass and larger shifts in fungal community composition than lower severity fires. Chapter 3 tested the mechanisms through which boreal forest fires alter decomposition processes. I discovered that decomposition rates were slower in recently burned forests because of post-fire reductions in soil moisture and C substrate quality. In the second part of my dissertation I expanded my findings to other types of disturbance events using meta-analysis. Chapter 4 reviewed the response of soil microbial biomass to fires. I found that soil microbial biomass was significantly lower in recently burned ecosystems, but the response of microbial biomass to fire differed by fire type and biome. Chapter 5 examined soil microbial responses to abiotic (fire, harvesting, storms) and biotic (insect infestation, pathogen outbreaks) disturbances in forests. I observed that abiotic disturbances significantly reduced soil microbial biomass, while changes in microbial biomass were non-significant following biotic disturbance events. Collectively, these findings suggest that reductions in soil microbial biomass and decomposition rates following abiotic disturbances are likely to slow the transfer of C from soils to the atmosphere and provide a negative feedback to rising atmospheric CO2 concentrations and global change.

The relationship between microbial community composition and soil stability was determined using rainfall simulation and microbial lipid analysis. Study sites were located on road cuts and ski runs, and were from either granitic or volcanic parent material. Phospholipid fatty acid analysis was performed on soil samples from all treatment plots to determine total microbial biomass and the relative abundance of the following microbial groups and biomarkers: gram-negative bacteria, gram-positive bacteria, actinomycetes, fungi, arbuscular mycorrhizal fungi, and stress indicator biomarkers. The influence of treatment on microbial groups was determined, and microbial groups were related to the occurrence of runoff and rainfall parameters. This study shows that two important factors contributing to improved soil infiltration include: increasing total microbial biomass on all sites, and utilizing amendments that select for fungi on severely degraded soils. These relationships can guide experimental work to further evaluate how microbes interact with substrate type and other soil treatments to improve infiltration.--adapted from abstract.