The atmospheric deposition of anthropogenic nitrogen (N) can substantially alter carbon (C) cycling in soils, which may feed back to influence the rate at which C accumulates in the atmosphere. Previously, we found that over twenty years of experimental atmospheric N deposition has reduced microbial decay and increased organic matter content in the forest floor (+51%) and mineral soil (+18%) of a long-term study spanning a latitudinal gradient in northern hardwood forests of Michigan, USA. Additionally, the biochemical composition of accumulated soil organic matter indicates it is derived largely from fine root litter, suggesting that the altered decay of fine roots by saprophytic microbes is driving this biogeochemical response to simulated N deposition. To test this hypothesis, we placed litter bags containing rinsed and dried fine roots from our field sites between the forest floor and mineral soil of our long-term field experiment, and collected them after 4 and 12 months. We used high throughput sequencing of the bacterial 16S rRNA gene to assess the influence of experimental N deposition on bacterial communities specifically involved in the decomposition of fine root litter.
Root litter bacterial communities responded significantly to N deposition at both time points (PERMANOVA; P < 0.05). Experimental N deposition significantly decreased the abundance of Bacteroidetes (-12%) and Gemmatimonadetes (-24%) after 4 months, whereas Actinobacteria (+41% at 4 months; +32% at 12 months) and Saccharibacteria (+90% at 4 months; +53% at 12 months) consistently increased across both time points (ANOVA; P < 0.05). A small proportion (~10%) of bacterial OTUs account for >50% of dissimilarity between ambient and N deposition communities; however, these OTUs represent all 17 of the most abundant phyla (>0.1%). Increased availability of different organic substrates due to reduced fungal decay is a plausible mechanism by which anthropogenic N deposition indirectly affects bacteria. Because organic substrate preference is conserved at fine taxonomic resolution, and our results indicate that the response to experimental N deposition is widely distributed among root-decaying soil bacteria, accounting for bacterial C-use traits at finer taxonomic resolution may be necessary to better understand soil C dynamics in this ecosystem. Together, our results suggest shifts in bacterial community composition, including an increased abundance of some taxa with lignolytic physiologies (e.g., Actinobacteria), may contribute to the accumulation of root-derived organic matter in northern hardwood forest soils.