Does microbial community composition and function parallel environmental gradients which structure landscape-level patterns of plant community composition and biogeochemical processes? Because soil microbial communities are in part structured by the biochemical composition of plant detritus and in part by environmental conditions, we reasoned that landscape-level patterns of microbial community composition and function should follow the spatial distribution of floristically and functionally distinct forest ecosystems. We tested this idea by studying replicate stands (n = 3) in three forest ecosystems that differ in plant community composition, litter biochemistry, organic matter decomposition, as well as soil C and N cycling. The forest ecosystems we studied have a broad geographic distribution in the Upper Great Lakes region, and replicate stands in each ecosystem type occurred across a 5000 km² area.  We quantified the composition of fungal and bacterial communities using a range of molecular techniques (PLFA, DGGE, T-RFLP, quantitative PCR) and assayed their metabolic capabilities using extracellular enzyme assays (i.e., cellobiohydrolase and phenol oxidase) coupled with the quantitative amplification of functional genes. The relative abundance of soil fungi within the microbial community, as well as fungal community composition and lignolytic activity, paralleled spatial patterns of plant litter biochemistry among ecosystems, especially differences in leaf litter lignin concentration. Our observations collectively support the idea that plant and microbial communities co-occur across the landscape, wherein compositionally and functionally distinct microbial communities occurred within floristically and biogeochemically distinct forest ecosystems.  Moreover, our results indicate that composition and function are linked in soil microbial communities, and this relationship in part controls the biogeochemical cycling of C and N across landscapes.