Do local-scale context dependencies shape how ectomycorrhizal fungal diversity structures with reduced or sustained experimental N addition?

Abstract

Atmospheric nitrogen (N) pollution visibly and rapidly changes forest systems, for example impacting the diversity of forest ectomycorrhizal (EcM) fungi. EcM fungi are measurable by components that vary in longevity, hence, reflecting different temporal scales of presence: EcM root-tips are viable for, at most, a couple growing seasons, compared to fungal structures that may persist in soil as DNA for decades or longer. In-growth mesh bags, in contrast, typically capture a single growing season of hyphal growth. The different components might portray different diversity responses by EcM fungi to large-scale N pollution, as well as any subsequent reductions from improved environmental standards. Within an established oak forest study system, we examined the impact of sustained and recently reduced experimental N addition on EcM fungal diversity and composition, measured in three main fungal components (root-free soil, root-tips, and mesh bag hyphae). We hypothesized that elevated soil N would reduce EcM fungal diversity, and that composition would change, but with differences among fungal components related to the temporal longevity of the components. Our expectations were largely met, in that richness primarily declined with increased soil N, and all trends were most pronounced with the soil EcM fungi (the only component potentially reflective of long-term fungal presence). We discovered that abatement of the experimental N treatment did not revert fungal trends to those of the same-site plots with ambient N treatment. Instead, the stochastic nature of local-scale disturbances, related to invasive earthworms and forest stand dynamics, likely impacted N levels and, thus, EcM fungal trends. Due to the context-dependency of localized disturbance(s), assessing the effects of reduced large-scale N deposition on EcM fungi can prove to be challenging.