In this dissertation, I examine how environment-altering anthropogenic disturbances affect the relationship between plants and beneficial soil microbes. Many disturbances change key aspects of the soil environment, directly affecting both plants and soil microbes, and potentially altering plant-microbe interactions. I use a combination of field and greenhouse experiments to compare plant-soil microbe interactions across different levels of environmental disturbance. In Chapter I and II, I used a well-known pollution gradient across montane forests in Southern California to examine how nitrogen (N) deposition alters the structure and function of ectomycorrhizal fungi in forests with a Mediterranean climate. Many studies have identified strong effects of anthropogenic nitrogen deposition on the community composition of ectomycorrhizal fungi in boreal and temperate forest soils. These ecosystems are typically N limited, and it is not clear whether N deposition has similar effects on ectomycorrhizal communities in less productive ecosystems where N may be colimiting to plant productivity along with other resources. I tested the effects of N deposition on ectomycorrhizal communities within forests receiving a range of N deposition in the San Bernardino National Forest of Southern California, where tree growth during the growing season is typically limited by water availability. To determine the effects of N deposition on ectomycorrhizal communities, I sampled both the ectomycorrhizae colonizing the root tips of ponderosa pine and present as fungal hyphae, collectively referred to as mycelium. Ectomycorrhizal fungi present in these samples were identified using PCR based molecular methods. In addition, I tested whether N deposition alters functional traits of ectomycorrhizal communities by assaying the production of a suite of extracellular enzymes that target a range of organic soil nutrients. For enzyme assays, I used ectomycorrhizal colonized root tips collected from forest stands across the N deposition gradient during both dry and wet seasons. Ectomycorrhizal community composition was correlated with N deposition for both colonized root tip and mycelium samples. In addition, mycelium abundance was negatively correlated with soil nitrate concentration. However, the function of ectomycorrhizal communities was not strongly affected by N deposition in this study, and N deposition was much less important to EMF enzyme production than seasonal effects. I found that enzyme production was consistent in most seasons across the N deposition gradient for enzymes that target organic nitrogen sources, organic phosphorus, and recalcitrant carbon sources. Only the production of glycoside hydrolyzing enzymes were positively correlated with N deposition, and only during the wettest sampling period in December. These results confirm that N deposition is changing ectomycorrhizal communities and abundance, even in dry forests where conditions may reduce the relative importance of nitrogen nutrition for forest trees. However, the change in community composition does not appear to result in a change in the capacity of EMF communities to produce extracellular enzymes. In Chapter III, I evaluate the effects of multiple mechanisms of plant soil feedbacks in exotic and native grasses using a greenhouse study in which soil microbial communities, soil organic matter distribution, and nitrification rates were manipulated. Both exotic and native grasses exhibited evidence of negative plant soil feedback, but the results were much stronger in native grasses. In addition, different mechanisms of plant soil feedback were important for exotic and native grasses. Mixing soil profiles, a treatment that redistributed soil resources throughout the soil column, caused exotic grasses to grow significantly more deep roots. As exotic grasses tend to concentrate roots and soil resources in upper soil layers, this result may indicate potential negative feedback under field conditions. Native grass biomass responded positively to sterilization and reduced nitrification. That different feedback mechanisms were important in explaining the plant soil feedback between co-occurring species highlights the value of testing multiple feedback mechanisms and underscores the diversity of changes that exotic species may have on the soil environment.