Fungi are mediators of the carbon (C) and nitrogen (N) cycles, two of the most important elements in terrestrial ecosystems. At Harvard Forest (HF), over 30 years of simulated N deposition results in soil organic matter accumulation. Previous studies at HF showed that elevated soil N favored stress-tolerant fungi with low potential for decomposition. However, the mechanisms behind this observation remain unknown; decreases in fungal biomass, shifts in fungal community composition, and evolutionary adaptation of fungi may all contribute.
Here, we explored the fungal mechanisms that shape ecosystem-scale organic matter accumulation at HF. We hypothesized that fungal functional groups with high gene frequency for N uptake would increase in abundance under long-term simulated N deposition and, that these fungal functional groups would have low C acquisition gene frequency. In addition, we tested patterns of C acquisition at different N concentrations in different saprotrophic filamentous fungi isolated from control and N amended soils. Our objective was to determine changes in C acquisition in response to different N concentrations to gain insight into C acquisition genes that may be selected for under elevated N conditions.
We quantified C and N gene frequencies of ~1000 fungal genomes from the public database MycoCosm. We grouped them into fungal functional groups with FunGuild and estimated their C acquisition and N uptake potential. In addition, we used soil RNA data from HF and calculated N response for each fungal functional group. We also isolated fungi from control and N amended soils and tested C acquisition changes of seven different C sources at two different N levels using MicroResp (i.e. microplate-based respiration system).
Results/Conclusions
We found support for our hypothesis because genome analysis paired with soil RNA data showed that fungal functional groups with high N uptake and low C acquisition gene frequency respond positively to soil N enrichment. But saprotrophic filamentous fungi, which have an intermediate N uptake gene frequency and species-dependent C acquisition gene frequency, vary in their response to N deposition. MicroResp showed that fungi isolated from N amendment plots were more efficient at acquiring certain C substrates (i.e. xylose, trehalose, butyric acid, and N-acetyl glucosamine) compared to fungi isolated from control plots. We conclude that exposure to elevated soil N is a selective force that favors fungi with elevated N uptake gene frequency, as well as saprotrophic fungi with the ability to carry out decomposition under high N conditions, thus allowing them to subsist.