Abstract: Bacteria are an abundant and physiologically diverse group of organisms critical to biogeochemical transformations in soils. Identifying the mechanisms that underlie patterns of diversity and community composition is critical for understanding how perturbations, such as changing land use and climate change, will affect these systems. My dissertation research is focused primarily on evaluating aluminum (Al) toxicity and Al- and Fe-bearing minerals as mechanisms structuring bacterial communities. Al is ubiquitous in soils and is generally non-toxic in crystalline mineral phases. However the dissolved trivalent Al ion is highly toxic to organisms even at sub-ppm levels. In Chapter 2, I characterize the bacterial communities along a lithosequence that forms a dissolved Al and pH gradient. Bacterial community structure in these soils varied as a function of both parent material and soil solution chemistry. In Chapter 3, I evaluate soil Al as a predictor of bacterial community composition and diversity at the landscape scale. Bacterial community structure was significantly, highly correlated with exchangeable Al, which was used as a proxy for the Al3+ ion, the dominant, toxic dissolved Al species in soils. In Chapter 3, I characterize the effects of two common Al and Fe oxides on decomposer communities. Community structure responded to solution chemistry associated with each treatment. In Chapter 4, I use H218 O-DNA stable isotope probing to characterize the response of bacteria to increased soil acidity. The short-term responses of specific taxa did not mirror responses observed in field-scale, survey-based studies. Notably Actinobacteria increased in abundance relative to other taxa while Acidobacteria showed no change in relative abundance.