Metagenomic characterization of soil microbial communities in the Luquillo experimental forest (Puerto Rico) and implications for nitrogen cycling

The phylogenetic and functional diversity of microbial communities in tropical rainforests, and how these differ from temperate communities remain poorly described but are directly related to the increased fluxes of greenhouse gases such as nitrous oxide (N2O) from the tropics. Towards closing these knowledge gaps, we analyzed replicated shotgun metagenomes representing distinct life zones from four locations in the Luquillo Experimental Forest (LEF), Puerto Rico. These soils had a distinct microbial community composition and lower species diversity when compared to temperate grasslands or agricultural soils. Unlike temperate soils, LEF soils showed little stratification with depth in the first 0-30cm, with ~45% of community composition differences explained solely by location. The relative abundances and nucleotide sequences of N2O reductases (nosZ) were highly similar between tropical forest and temperate soils. However, respiratory NO reductase (norB) was 2-fold more abundant in the tropical soils, which might be relatable to their greater N2O emissions. Nitrogen fixation (nifH) also showed higher relative abundance in rainforest compared to temperate soils (20% vs. 0.1-0.3% of bacterial genomes in each soil type harbored the gene, respectively). Collectively, these results advance our understanding of spatial diversity and metabolic repertoire of tropical rainforest soil communities, and should facilitate future ecological modeling efforts. ImportanceTropical rainforests are the largest terrestrial sinks of atmospheric CO2 and the largest natural source of N2O emissions, two critical greenhouse gases for the climate. The microbial communities of rainforest soils that directly or indirectly, through affecting plant growth, contribute to these fluxes remain poorly described by cultured-independent methods. To close this knowledge gap, the present study applied shotgun metagenomics to samples selected from 3 distinct life zones within the Puerto Rico rainforest. The results advance our understanding of microbial community diversity in rainforest soils and should facilitate future studies of natural or manipulated perturbations of these critical ecosystems.