Contrasting effects of warming and simulated nitrogen deposition on standing and active fungal communities and their functioning in litter decomposition in a temperate hardwood forest

Background/Question/Methods

Litter decomposition is the first step in the transformation of plant litter to CO₂, stable soil organic matter, or microbial biomass, and fungi are the primary producers of the extracellular enzymes that drive that process. Anthropogenic nitrogen (N) deposition and warming of soils due to climate change are projected to escalate in the future, and have been shown to alter decomposition dynamics. We examined the responses of standing and active fungal communities, and extracellular enzyme activities (EEA), in decomposing litter exposed to soil warming and simulated N deposition. We expected N additions to decrease and warming to increase litter decay rates over two years of decomposition, and expected these changes to be accompanied by shifts in EEA. Furthermore, we expected N additions to decrease richness of the active fungal community and the relative abundance of basidiomycetes, in particular. We expected warming to favor ascomycetes after one year of decomposition with a shift to basidiomycetes after two years. Mass loss, moisture, total carbon (C) and N, and EEA were measured after one and two years of litter decomposition. Standing and active fungal communities were measured by pyrosequencing the fungal ribosomal internal transcribed spacer, which was PCR-amplified from total RNA and DNA.

Results/Conclusions

Decay rate was decreased by N addition over two years of decomposition, while warming had no effect on decay rate. Phenol oxidase EEA responded negatively to N addition but positively to warming. EEA of cellulolytic enzymes responded positively to N addition, and peroxidases responded negatively. Both warming and N additions caused shifts in fungal community composition. Warming and the N treatments had opposite effects on the richness of active fungi relative to the standing community, with N additions having a negative effect, and warming, a positive one. The relative abundance of active ascomycetes was depressed relative to standing ascomycetes by high levels of N addition, while warming decreased the relative abundance of active basidiomycetes. The effects of warming and N additions on decay rate and EEA generally confirmed our hypotheses, and N additions suppressed richness of the active fungal community as we expected. However, the apparent suppressive effect of warming and N additions on the activity of basidiomycetes, and ascomycetes, respectively, was unanticipated. The contrasting effects of warming and N additions suggest that continued global change will disrupt normal C cycling through litter decay by altering the fungal community, and that the interaction of these effects may be complex.