Selection among lower taxonomic ranks of bacteria in tropical forest decomposer communities

Tropical forest ecosystems are often rooted in weathered soils, with most nutrients trapped in the overlying biomass. Decomposer communities, largely comprised of microorganisms, quickly recycle nutrients that fall to the forest floor. Multiple nutrients limit rates of decomposition in the forests of Barro Colorado Nature Monument, Panama. Furthermore, bacterial, but not fungal, decomposer communities responded to 9 years of factorial N, P, and K amendment as shown by shifts in composition (taxa present) or structure (relative abundance of taxa). Here, we use deep sampling via pyrosequencing of bacterial 16S rRNA libraries across fertilized plots (n = 32) to identify shifts in relative abundance of bacterial populations. Community composition responded to fertilization at lower but not higher taxonomic resolutions. Subphyletic groups 1, 2, and 3 of the Acidobacteria differentiated on +N and +P plots; families of the Bacteroidetes (namely Chitinophagaceae) increased on +K plots, and families of the Myxococcales differentiated on +P plots. Finally, sequences of functionally grouped nitrifying genera Nitrobacter, Nitrospira, and Nitrosospira, although scarce, increased as predicted on +N plots. Deep sampling with pyrosequencing at various taxonomic ranks, combined with replicated, factorial fertilization experiments is an effective means of identifying potential mechanisms driving the structure of bacterial decomposer communities.