SYNTHESIS AND EMERGING IDEAS Mycorrhizal associations of dominant trees influence nitrate leaching responses to N deposition

Temperate forests receive some of the highest rates of nitrogen (N) deposition in the world. While numerous studies have investigated the effects of N enrichment on forests, there is little consensus on why some forests become N saturated while others do not. To investigate this, we used a multi-factor metaanalysis to simultaneously estimate the relative importance of several environmental, experimental, and anthropogenic variables on nitrate (NO3 ) leaching in response to experimental N addition. Given that overstory tree species composition and soil C:N ratio influence forest responses to N, we hypothesized that forests dominated by arbuscular mycorrhizal (AM) trees would respond differently than forests dominated by ectomycorrhizal (ECM) trees in the context of forest susceptibility to NO3 leaching. We found that mycorrhizal association is an important predictor of NO3 leaching, and AM-dominated forests leach more NO3 in response to N deposition than ECM forests. Additionally, we found that the amount of total N added, ambient N deposition rates, and the form of N added influenced the magnitude of the NO3 leaching response. Given that the mycorrhizal associations of most temperate trees are known, our results suggest that this functional grouping may be useful in identifying forests that are most susceptible to NO3 leaching.