Global changes will modify future nutrient availability with implications for grassland biogeochemistry. Soil organic matter (SOM) is central to grasslands for both provision of nutrients and climate mitigation through carbon (C) storage. While we know that C and nitrogen (N) in SOM can be influenced by greater nutrient availability, we lack understanding of nutrient effects on C and N coupling and stability in soil. Different SOM fractions have different functional relevance and mean residence times, i.e., mineral-associated organic matter (MAOM) has a higher mean residence time than particulate organic matter (POM). By separating effects of nutrient supply on the different SOM fractions, we can better evaluate changes in soil C and N coupling and stability and associated mechanisms. To this end, we studied responses of C and N ratios and distributions across POM and MAOM to 6–10 years of N, phosphorus (P), potassium and micronutrients (K_+µ), and combined NPK_+µ additions at 11 grassland sites spanning 3 continents and globally relevant environmental gradients in climate, plant growth, soil texture, and nutrient availability. We found addition of N and NPK_+µ generally reduced C:N in MAOM and POM. However, at low fertility and at warm, sandy sites, nutrient addition promoted higher MAOM and POM C:N, respectively. Addition of NPK_+µ also promoted C storage in POM relative to MAOM, and this was consistent across sites. Our results suggest that addition of macro- and micronutrients consistently decrease SOM stabilization, whereas responses of soil C:N stoichiometry were contingent on SOM fraction and environmental conditions.

全球变化将改变未来的养分可用性，并对草原生物地球化学产生影响。土壤有机质 (SOM) 是草原的核心，可通过碳 (C) 储存提供养分和减缓气候变化。虽然我们知道 SOM 中的 C 和氮 (N) 会受到更大的养分可用性的影响，但我们对养分对土壤中 C 和 N 耦合和稳定性的影响缺乏了解。不同的 SOM 组分具有不同的功能相关性和平均停留时间，即矿物相关有机物 (MAOM) 的平均停留时间高于颗粒有机物 (POM)。通过分离养分供应对不同 SOM 组分的影响，我们可以更好地评估土壤 C 和 N 耦合和稳定性的变化以及相关机制。为此，_+µ )，并在横跨 3 大洲的 11 个草原地点和全球相关的气候、植物生长、土壤质地和养分供应环境梯度中结合了 NPK _{+µ添加量。}我们发现添加 N 和 NPK _+µ通常会降低 MAOM 和 POM 中的 C:N。然而，在低肥力和温暖的沙地，养分添加分别促进了更高的 MAOM 和 POM C:N。相对于 MAOM，添加 NPK _+µ也促进了 POM 中的 C 存储，这在各个站点之间是一致的。我们的结果表明，添加大量和微量营养素会持续降低 SOM 的稳定性，而土壤 C:N 化学计量的响应取决于 SOM 分数和环境条件。