Bacterial community structure and assembly dynamics hinge on plant litter quality.

Litter decomposition is a fundamental ecosystem process controlling the biogeochemical cycling of energy and nutrients. Using a 360-day lab incubation experiment to control for environmental factors, we tested how litter quality (low C/N deciduous versus high C/N coniferous litter) governed the assembly and taxonomic composition of bacterial communities and rates of litter decomposition. Overall, litter mass loss was significantly faster in soils amended with deciduous (DL) than coniferous (CL) litter. Communities degrading DL were also more taxonomically diverse and exhibited stochastic assembly throughout the experiment. In contrast, alpha-diversity rapidly declined in communities exposed to CL. Strong environmental selection and competitive biological interactions induced by molecularly complex, nutrient poor CL was reflected in a transition from stochastic to deterministic assembly after 180 days. Constraining how the diversity and assembly of microbial populations modulates core ecosystem processes, such as litter decomposition, will become increasingly important under novel climate conditions, and as policy makers and land managers emphasize soil carbon sequestration as a key natural climate solution.