Nitrogen (N) and phosphorus (P) additions to grasslands increase aboveground plant biomass and modify plant community composition, thereby affecting plant‐derived organic carbon (C) input to soil and soil C cycling and storage. However, the effects of N and P additions on soil organic C (SOC) decomposition and sequestration are not fully understood and their underlying mechanisms are poorly known. This study aimed to explore the mechanisms underlying SOC decomposition and SOC content decline in the topsoil of alpine meadows on the Tibetan Plateau after 9‐years of field N and P additions. Stoichiometric characteristics of soil and microorganisms and their effects on microbial decomposition, including priming effects (PEs), C substrates decomposition, and microbial C use efficiency (CUE), were investigated by adding ¹³C labeled substrate (glucose or vanillin). Results showed that N and P additions differentially affected the magnitude and direction of PEs and SOC decomposition, accelerated mineralization of glucose and vanillin by 33–45% and 11–45%, respectively, but decreased microbial CUE of glucose and vanillin by 9–15% and 11–48%, respectively. These effects were caused by differential increase in microbial activity and acceleration of microbial decomposition due to N and P additions, and induced lower soil ecological stoichiometric ratios and higher microbial C:N:P ratios. The above effects led to different magnitudes of decomposition and accumulation of SOC and plant‐derived C substrate and thus decline in SOC content depending on N and P additions. We found that long‐term N and P additions would weaken soil functioning as a C pool for alpine meadows on the Tibetan Plateau.

中文翻译：

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氮磷添加量对青藏高原高寒草甸土壤有机碳分解和积累的影响

草地中氮（N）和磷（P）的添加增加了地上植物的生物量并改变了植物群落的组成，从而影响了植物衍生的有机碳（C）输入土壤和土壤C的循环和存储。然而，氮和磷的添加对土壤有机碳（SOC）的分解和固存的影响尚不完全清楚，其潜在机理尚不清楚。这项研究旨在探讨在添加氮和磷9年后，青藏高原高寒草甸表层土壤SOC分解和SOC含量下降的潜在机制。通过添加¹³，研究了土壤和微生物的化学计量特征及其对微生物分解的影响，包括启动效应（PEs），碳底物分解和微生物碳利用效率（CUE）。C标记的底物（葡萄糖或香草醛）。结果表明，氮和磷的添加对PE和SOC分解的大小和方向有不同的影响，分别使葡萄糖和香兰素的矿化速率提高33-45％和11-45％，但使葡萄糖和香兰素的微生物CUE降低9-15 ％和11–48％。这些影响是由于微生物活动的差异性增加以及由于氮和磷的添加而加速的微生物分解，以及导致较低的土壤生态化学计量比和较高的微生物C：N：P比值所致。上述影响导致SOC和植物来源的C底物分解和积累的程度不同，因此，根据N和P的添加，SOC含量下降。