Impacts of nitrogen deposition on forest mycorrhizal communities

Humans have dramatically increased atmospheric nitrogen (N) deposition globally. This increase in N deposition is correlated with shifts from ectomycorrhizal to arbuscular mycorrhizal tree dominance and with the decline or disappearance of key taxa of ectomycorrhizal fungal (EcMF) and arbuscular mycorrhizal fungal (AMF) communities. Strong plant-soil microbe-soil feedbacks may slow recovery of forest mycorrhizal communities after abatement of N deposition. Current estimates of critical loads are 5–6 kg N ha−1 year−1 for conifer-associated EcMF and 10–20 kg N ha−1 year−1 for broadleaf temperate EcMF; other critical loads require additional study. Responses of EcMF to N deposition are better understood than those of AMF, largely because the former are easier to identify. Functional consequences of EcMF community change are linked to decreases of fungi with medium-distance exploration strategies, hydrophobic walls, proteolytic capacity, and perhaps peroxidases for acquiring N from soil organic matter. These functional losses may contribute to declines in forest floor decomposition under N deposition. For AMF, limited capacity to directly access complexed organic N may reduce functional consequences, but research is needed to test this hypothesis. Mycorrhizal biomass often declines with N deposition, but the relative contributions of soil versus host mediated mechanisms for this decline have not been quantified. Furthermore, fungal biomass, fungal stoichiometry (e.g., C:N:P), and functional responses to N inputs probably depend on ecosystem P status, but the interactions between N deposition-induced P limitation, belowground C flux and mycorrhizal community structure and function are still unclear. Current omic analyses indicate potential functional differences among fungal lineages that require integration with measured functional and ecosystem consequences of N-mediated changes in mycorrhizal fungal community composition and structure in both laboratory and field studies.