Nitrogen deposition and water addition affect microbial carbon and nitrogen cycling: Insights from a 10-year experiment in a semi-arid steppe

Anthropogenic nitrogen (N) deposition and increased precipitation are known to alter soil microbial communities. However, the interactions between N deposition and increased precipitation on soil microbial composition and the microbial-mediated carbon (C) and N cycling functions remain unclear. A 10-year experiment was conducted on the Mongolian Plateau to evaluate the effects of N deposition (0, 10 g N m⁻² year⁻¹) and water addition (0%, 30% above average annual precipitation) on: (1) soil nutrient dynamics and pH, and (2) their subsequent impacts on bacterial community structure and C/N cycling functions. We found that long-term N deposition significantly altered the bacterial community composition, specifically increased the abundance of Alphaproteobacteria, Firmicutes, and Bacteroidetes while reducing Acidobacteria. It also simplified bacterial network complexity and stimulated C metabolic functions (aromatic compound degradation, cellulolysis, hydrocarbon degradation). Water addition enhanced functions linked to chitin degradation, aromatic compound degradation and nitrogen fixation. Structural equation modeling suggests N deposition indirectly promoted C cycling via soil acidification, while water addition directly boosted N cycling and indirectly influenced it through soil organic carbon. Our findings underscore that N deposition will enhance bacterial C acquisition and emphasize the importance of pH in regulating microbial nutrient cycles. This study provides new experimental insights into the potential mechanisms by which nutrient inputs and precipitation changes affect microbial community structure and functions of the C and N cycling.众所周知，人为氮沉降（Anthropogenic nitrogen deposition）与降水增加会改变土壤微生物群落。然而，氮沉降与降水增加对土壤微生物群落组成，以及微生物介导的碳（C）、氮（N）循环功能的交互影响仍不明晰。本研究在蒙古高原开展了为期10年的野外控制实验，旨在评估氮沉降（设置梯度：0、10 g N m⁻²·年⁻¹）与增水处理（设置梯度：0%、较年均降水量增加30%）对以下两方面的影响：(1) 土壤养分动态与pH值；(2) 二者后续对细菌群落结构及碳氮循环功能的作用。研究结果表明，长期氮沉降显著改变了细菌群落组成：具体表现为增加了α-变形菌纲（Alphaproteobacteria）、厚壁菌门（Firmicutes）与拟杆菌门（Bacteroidetes）的相对丰度，同时降低了酸杆菌门（Acidobacteria）的丰度。此外，长期氮沉降还简化了细菌共生网络的复杂度，并激活了碳代谢相关功能（如芳香族化合物降解、纤维素分解、烃类降解）。增水处理则强化了与几丁质降解、芳香族化合物降解及固氮作用相关的微生物功能。结构方程模型（Structural equation modeling）分析显示，氮沉降通过土壤酸化间接促进了碳循环；而增水处理则直接推动了氮循环，并通过土壤有机碳对其产生间接影响。本研究结果证实，氮沉降将提升细菌的碳获取能力，同时凸显了pH值在调控微生物养分循环中的关键作用。本研究为解析养分输入与降水变化如何调控微生物群落结构及碳氮循环功能的潜在机制提供了全新的实验依据。