Technical Abstract: Aim Understanding the drivers of community composition across spatial scales is of keen interest to ecologists. Although patterns are emerging for macroorganisms, we only have a basic understanding of the factors determining soil microbial community composition, diversity, and productivity at large spatial scales. We hypothesized that factors associated with land use and climate would be the primary drivers of the composition of soil microbial communities and their diversity and biomass (i.e., proxy for microbial productivity). Methods Using phospholipid fatty acid analysis (PLFA), we compared microbial communities across eight land use types (coastal grasslands, inland grasslands, deserts, perennial agricultural fields, annual agricultural fields including flooded rice fields, coniferous forests, and freshwater wetland soils) sampled throughout the State of California, USA (n=1,117). Results The two major factors driving composition, diversity, and microbial biomass were land use type, especially related to water availability, and disturbance. Dry soils tended to be more enriched in Gram negative bacteria and fungi, and wetter soils were more enriched in Gram positive, anaerobic, and sulfate-reducing bacteria. Although diversity was highest in the ecosystems with the wettest soils, microbial biomass, based on total PLFA, was lowest in the ecosystems with the wettest and driest soils. Additionally, disturbed soils tended to have less fungal biomass and more Gram positive bacterial biomass than wildland soils. Surprisingly, some factors known to shape microbial communities, such as soil pH and specific plant taxa, did not influence their ordination. Instead, land use type and disturbance were the strongest factors influencing microbial communities at the regional scale (i.e., State of California). Main conclusions Distinct communities, structured by land use and climatic factors, were apparent in this regional analysis. Differing from regional and global patterns for plants and animals, soil microbial community composition was little affected by gradients in latitude, elevation, and precipitation. Strong effects of other environmental and management factors on microbial community composition suggest that inclusion of agroecosystems divorced these communities from gradients in latitude and precipitation due to the effects of factors such as tillage, flooding, and irrigation. Given the nearly asymptotic relationship between microbial biomass (i.e., proxy of productivity) and diversity (i.e., number of fatty acids), we suggest considerable functional redundancy may be present in these communities, such that as microbial niches become saturated, increasing productivity does not always result in increasing diversity.