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

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. 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. 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 1.37× higher in +N soils than controls; correspondingly, fungal necromass C contributed 1.42× 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. We propose that N deposition increases SOC storage by enhancing saprotroph abundance and activity, whereby increased organic acid production supports fungal growth both directly and indirectly via the release of nutrients and DOC, resulting in greater inputs of necromass to the soil. Our findings highlight the importance of considering shifts in guild‐level fungal abundance and function to investigate changes in SOC dynamics and storage. Read the free Plain Language Summary for this article on the Journal blog. 氮沉降增强真菌驱动热带森林土壤有机碳固存的潜力及其机制 自20世纪以来，由于工业的发展和农牧业的扩张，大气氮排放量远超过自然生物固氮量，导致大气氮沉降水平迅速上升。过量的氮输入深刻影响并改变了植物凋落物向土壤有机碳的转化过程，进而调控热带土壤有机碳的动态和固持。真菌是凋落物的主要分解者，驱动植物源碳向土壤的转运，因此，弄清真菌群落对氮沉降的响应有助于理解热带森林土壤有机碳动态变化的机制。依据不同生态功能，真菌群落可划分为不同生态功能群(guilds)。这些功能群通常对相同的环境变化有截然不同的反应，但是，关于在氮沉降背景下，热带森林土壤中不同生态功能群与相关功能潜力的变化及其与土壤有机碳动态的关系等认识仍不清晰。基于中国科学院小良研究站长期模拟氮沉降试验样地，本研究探讨了氮沉降对热带森林真菌群落、功能潜力和土壤碳组分的影响。研究发现，施氮土壤的总有机碳、溶解性有机碳、颗粒态有机碳和真菌残