Enhanced abundance of generalist and litter saprotrophs explain increased tropical forest soil carbon with long‐term nitrogen deposition

1. Nitrogen (N) deposition is a pervasive anthropogenic change that can alter the dynamics and storage of carbon (C) in tropical soils by altering the transformation of plant litter C to soil organic C (SOC). The response of fungi may be particularly important for explaining how N deposition affects SOC storage in tropical forests because they are the primary decomposers of plant litter. Fungi can be grouped into different ecological guilds, or functional groups, which often have contrasting responses to the same environmental change. However, little is known if shifts in fungal guilds and their functions explain increased tropical forest SOC storage under N deposition. 2. We addressed this knowledge gap by assessing the effects of long--term (6 years) experimental N addition (+N) on fungal guilds, their functions and SOC fractions in a tropical forest. 3. Total SOC, dissolved organic C (DOC), particulate organic C and fungal necromass C were all higher in +N soils. The fungal community in +N soils was dominated by generalist saprotrophs, and leaf saprotroph abundance increased nearly 93--fold compared to controls. These changes were accompanied by an increase in lignocellulolytic enzymes, indicating accelerated decomposition of recalcitrant litter C compounds. Chitosanase, which catalyses synthesis of glucosamine, was 1.37x higher in +N soils than controls; correspondingly, fungal necromass C contributed 1.42x more to SOC. Greater abundance of saprotrophic enzymes involved in organic acid synthesis in +N soils was associated with higher concentration of Ca2+ and DOC.