Carbon (C) cycling in terrestrial ecosystems is driven by the decomposition of plant litter, a process that is mediated by saprotrophic microbial communities. Increased nitrogen (N) deposition alters microbial community composition and decreases activity, often with a concurrent increase in soil organic matter (SOM). Most studies that examine saprotrophic communities focus on plant leaf decay, but recent evidence has suggested that fine roots may be a dominant source of SOM in soils. In our long-term experimental N deposition study, we have seen an increase in SOM (+18%) which is biochemically similar to fine roots. To test whether saprotrophic communities that decay roots are altered by N deposition, we incubated bags containing dead fine roots in our experimental N deposition sites and harvested them at 4 and 12 months. We then extracted the DNA and used high throughput sequencing to examine the fungal communities using the 28S rRNA gene.
Fungal communities from the root incubation bags responded significantly to experimental N deposition at both time points (PERMANOVA; P < 0.004, P<0.003), wherein richness and diversity generally increased under experimental N deposition. Six fungal classes represented the majority of the fungal communities, with Agaricomycetes alone accounting for an average of ~40% of the sequences. Three of these classes responded significantly to experimental N deposition at 4 months: Eurotiomycetes (+2.6), Agaricomycetes (-12%), and Tremellomycetes (+4%); only Tremellomycetes significantly increased (+6%) after 12 months.
Our results suggest that shifts from experimental N deposition in fungal taxa that decompose roots may have long-lasting consequences for C cycling and storage. As Agaricomycetes represented the overwhelming majority of our fungal community, this group may play a key role in root decomposition. Furthermore, it was negatively impacted by experimental N deposition, indicating it may be important for SOM accumulation as anthropogenic N deposition continues into the future.