Contrasting influences of ectomycorrhizal fungi on decomposition depending on soil fertility.

Despite their dominance in many terrestrial ecosystems, the impact of ectomycorrhizal fungi on organic matter decomposition is notably variable across different ecological settings. Here we outline how the influence of ectomycorrhizal fungi on decomposition of belowground organic matter pools depends, not only on the traits of the ectomycorrhizal fungi, but also on the relative dominance of other interacting fungal guilds, in a manner that is predictable across wide gradients in soil fertility. Specifically, we propose that the dominant effect of ectomycorrhizal fungi shifts from 'priming' of saprotrophs in the most fertile soils, to inhibition of saprotrophs when conditions become nitrogen-limited, and to active 'mining' of organic matter when nutrient availability is severely constrained. Thus, ectomycorrhizal fungi retard decomposition of organic matter at intermediate soil fertility but accelerate decomposition both in the most fertile and the least fertile soils. These ideas may be applied both on local fertility gradients and at the global biome scale, helping to reconcile inconsistent relationships between ectomycorrhizal fungi and soil organic matter dynamics.