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Soil bacteria and fungi play a central role in the biogeochemical cycling of both carbon (C) and nitrogen (N) through terrestrial ecosystems. In the C cycle, soil microbial groups regulate the depolymerization of large stocks of soil organic matter and contribute 35-69 Pg C to the atmosphere annually through heterotrophic respiration. Soil microbial groups also mediate several important transformations of N, including making limiting nutrients available for uptake by plants through N-fixation, converting N between inorganic forms through nitrification, and returning N to the atmosphere through denitrification. While each of these functions is performed by soil microbes, scaling microbial physiology and community structure to biogeochemical cycling remains a significant research challenge. This dissertation integrates three distinct approaches to characterizing relationships between microbial physiology, microbial community structure and biogeochemical cycling. First, I explore the role of microbial physiology in C cycling by developing a novel method to predict bacterial carbon use efficiency (CUE) from genomes using metabolic modeling. I find that bacterial CUE is phylogenetically structured, with the class and order levels explaining the greatest proportion of variance in CUE, and I identify particular bacterial traits that most strongly predict CUE. These findings highlight the importance of accounting for microbial physiology when modeling soil C cycling. Second, I explore how differences in the abundance and activity of microbial functional groups and their interactions with mycorrhizal fungi impact temperate forest N cycling. I find that N availability and rates of N-fixation, nitrification and denitrification are structured in relation to mycorrhizal fungal types, but that the abundances of bacterial functional groups are not correlated with biogeochemical fluxes. Finally, I use a soil biogeochemical model to identify sources of uncertainty and data needs in advancing our understanding of microbially-mediated soil biogeochemical cycling. I isolate specific microbial physiological and enzyme kinetic parameters that have disproportionately large impacts on projections of coupled C and N cycling, and I quantify the potential for particular types of data to help reduce uncertainties. Overall, this dissertation advances our understanding of how microbial processes impact the biogeochemical cycling of C and N in terrestrial ecosystems.