Abstract
In theory, ectomycorrhizal (EM) and saprotrophic fungi compete for nitrogen (N) found in soil organic matter. However, both positive and negative effects of EM fungi on decomposition have been observed across systems, with opposing implications for soil carbon (C) storage. The conditions driving the context dependency of fungal guild interactions remain poorly understood, which has limited our ability to predict the effects of EM fungi on biogeochemical cycling at regional and global spatial scales. To address this knowledge gap, we used a publicly available dataset of soil fungal communities to examine global patterns of relative EM and saprotrophic abundance and their influence on soil carbon and nutrient cycling. We demonstrate that EM fungal dominance and its effects on C and N cycling are predictable across the globe using only soil C : N stoichiometry, host tree functional group, and soil pH as predictors. We argue that because soil pH influences the availability and enzymatic catabolism of organic N, it determines the dominant N acquisition strategy of EM fungi, which in turn governs the directional effect of EM-saprotroph interactions on rates of organic matter decomposition in forests.