Functioning of decomposer fungi in a changing climate

Background/Question/Methods

Fungi are ubiquitous in terrestrial ecosystems and play an important role in biogeochemical cycling because of their function as litter decomposers. It has been demonstrated that increased nitrogen (N) deposition decreases fungal biomass and changes relative abundance of particular fungal groups and species. To understand the relationship between fungal community dynamics and litter decay in response to climate change we made use of an existing long-term study in which a northern hardwood forest has been exposed to simulated increased N deposition for 25 years (Chronic Nitrogen Addition plots, Harvard Forest, Petersham, MA). The experiment consists of three treatments (ambient, 5 and 15 g N m^-2 y^-1). A litterbag study was implemented in 2010 and replicate sets of litterbags were harvested after one and two years of decomposition. Mass loss, C:N, lignin/cellulose and various enzyme activities were measured on both sets of litterbags, and in addition, fungi were cultured from the decomposed litter. We measured growth rates and decomposition ability under different lab N conditions (representing field N availability) for a subset of cultured fungal species.

Results/Conclusions

In both years litter mass loss was reduced under N addition, with most of the mass loss observed in the first year compared to the second year (70% and 30% of total mass loss, respectively). Both years showed either increased activity for some hydrolytic enzymes (e.g. cellobiohydrolase) or no difference (e.g. ß-N-acetylglucosaminidase) with increased N, while the oxidative enzymes (e.g. peroxidases) showed no difference in activity in the first year, but had a highly reduced activity in year 2 with increased N. Some fungal species cultured from the litter showed reduced growth rate under increased medium N, while other species were not affected by the different media N concentrations. The decomposition experiment with the fungal cultures resulted in decreased decomposition rates with several fungal species isolated from the high N treatment compared to fungal isolates from the control treatment. The reduced mass loss of the litterbags under increased N addition in combination with different growth and decomposition rates of fungi isolated from the high N treatment indicates that even if fungal species persist in a high N environment, their functioning might be affected, which in turn can affect the functioning of the ecosystem they inhabit.