Contributions of ectomycorrhizal fungi to organic matter formation and decomposition in response to chronic N deposition

Background/Question/Methods

Due to intensified anthropogenic activities, atmospheric nitrogen deposition rates are likely to increase into the foreseeable future. In temperate forests, chronic N deposition promotes C storage in soil by altering organic matter decomposition processes and decomposer communities. Ectomycorrhizal fungi contribute and actively degrade organic matter and therefore must be considered for their role in soil organic matter dynamics. Plots within a temperate mixed-hardwood stand at the Harvard Forest Long-Term Ecological Research (LTER) site in Petersham, MA, USA have been continuously amended with ammonium nitrate at 50 or 150 kg N ha^-1 yr^-1since 1988 to simulate projected N deposition levels or N saturation, respectively. We determined how chronic N deposition affects organic matter inputs by ectomycorrhizal fungi by collecting hyphae in buried, root-excluded sand bags. We assessed the potential for organic matter decomposition by the ectomycorrhizal fungal community by measuring activity of five C, N, or P hydrolases and two oxidases at the mantle of ectomycorrhizae excised from organic and mineral soil horizons. We then identified the fungal symbiont of a random subset of ectomycorrhizae by DNA sequencing.

Results/Conclusions

Extraradical hyphal production was reduced at low and high rates of N deposition. Under low and high N deposition rates, ectomycorrhizal fungi had increased cellulolytic and chitinolytic activity in the organic soil horizon and reduced proteolytic activity in the mineral soil horizon, agreeing with earlier observations in bulk soil at this and other sites subjected to elevated N deposition. These parallels suggest that chronic N deposition exerts similar controls on decomposition by ectomycorrhizal fungi as the total decomposer community. Vertical stratification of enzyme production by ectomycorrhizal fungi was less pronounced than the effects of N; only chitinase activity was less in mineral soil compared to the overlying organic soil. Species richness of ectomycorrhizal fungi was least at the highest N deposition rate. Russula and Lactarius spp. increased in relative abundance under low and high N deposition, apparently at the expense of Cortinarius, Tomentella, and Tricholoma spp. that were only detected in plots without added N. Continued efforts in determining the fate of organic matter pools contributed and decomposed by ectomycorrhizal fungi communities will help to clarify their role in soil organic matter dynamics under elevated N deposition.