Litter decomposition is a globally important and enzymatically-complex biogeochemical process. It is mediated by a diverse assemblage of saprophytic microorganisms, and can be suppressed by anthropogenic N deposition. In a northern hardwood forest ecosystem in Michigan, USA, twenty years of experimentally increased N deposition has reduced forest floor decay and increased soil C storage (+18%). This ecosystem-level response occurred concomitantly with compositional changes in saprophytic fungi and Bacteria. Here, we paired extracellular enzyme assays with shotgun metagenomics to determine if anthropogenic N deposition has altered the functional potential of microbial communities inhabiting decaying forest floor in our experiment. We used ribosomal and functional gene databases (i.e., RDP and Subsystems) to gain insight into whether experimental N deposition altered the taxonomic composition, as well as the metabolic capacity of the soil microbial community. Further, sequence homology with manually curated gene databases was also used to explore the impact of anthropogenic N deposition on the genetic capacity of both bacteria and fungi to metabolize lignocellulose.
Results/Conclusions
Experimental N deposition reduced lignocellulolytic enzyme activity and changed the metagenomic composition of the microbial community inhabiting decaying forest floor, providing indirect evidence that compositional shifts elicited a functional response in our long-term field experiment. The activity of extracellular enzymes mediating plant litter and humus decay, cellobiohydrolase and peroxidase, were ~50% lower under experimental N deposition (P < 0.05). The metagenomes were dominated by the bacterial phyla Actinobacteria (~46%), Proteobacteria (~26%), and Bacteroidetes (~11%); however, the taxonomic composition of the soil microbial community was not affected by experimental N deposition. Experimental N deposition significantly affected the relative abundances of genes associated with 58 of 177 pathways within the Carbohydrates, Metabolism of Aromatic Compounds (MAC), and Respiration Subsystems level 3 classifications, 44 of which increased in abundance between the ambient and experimental N deposition treatment (adjusted P < 0.05). Experimental N deposition affected the composition of the Carbohydrates (P = 0.05), but not MAC or Respiration Subsystems classifications. However, neither the abundance nor composition of fungal lignocellulolytic genes was affected by experimental N deposition. Results presented here provide evidence that changes in the functional capacity of saprotrophic soil microorganisms mediate how anthropogenic N deposition increases the storage of C in soil.