Organic nitrogen addition suppresses fungal richness and alters community composition in temperate forest soils

Human-driven increases in bioavailable nitrogen over the last century have revealed the critical need to understand how nitrogen availability influences terrestrial ecosystems. In contrast to widespread evidence that increasing inorganic nitrogen strongly influences the diversity and composition of plant and microbial communities, the effect of organic nitrogen addition, the predominant form of nitrogen in soils, remains less clear. In this study, we conducted a field experiment manipulating both the amount and composition of soil organic nitrogen present in soil fungal in-growth bags, followed by molecular characterization of fungal communities after a four-month incubation. Saprotrophic and ectomycorrhizal fungi readily colonized experimental in-growth bags, indicating a broad overlap in the fundamental niches of these two functional guilds. Increases in labile forms of organic nitrogen resulted in sharp declines in species richness and diversity across fungal guilds, as well as notable shifts in fungal community composition. The relative abundance of fungi classified as molds and yeasts peaked where organic nitrogen was both high and most labile, whereas slower-growing saprotrophic and ectomycorrhizal fungi were more abundant in the non-amended treatments. Taken together, our results indicate that similar to inorganic nitrogen, increasing the amount of organic nitrogen can dramatically alter the richness and composition of fungal communities in temperate forest soils.