Abstract: The stability of soil organic carbon (SOC) is a key indicator for assessing the carbon sink function of alpine grasslands, and its dynamics are regulated by the biogeochemical cycles of nitrogen (N) and phosphorus (P). However, the response mechanisms of different carbon components (such as particulate organic carbon (POC) and mineral-associated organic carbon (MAOC)) in alpine grasslands to N and P additions remain unclear. A field experiment was conducted to investigate the response mechanisms of different soil carbon components to N and P additions in an alpine grassland of the Qinghai-Tibet Plateau. The results showed that N addition significantly increased SOC content in the surface soil (0-10 cm) and that the combined N and P addition significantly increased SOC reserves. Both N and P addition caused a significant decrease in soil pH. Single N addition and the combined addition of N and P significantly increased the EC, ASi content, and NH₄⁺-N content in the topsoil (0-10 cm). In the subsurface soil (10-20 cm), N and P addition had no significant effect on most basic soil physical and chemical properties, except for soil SOC and ASi content. Basic soil physical and chemical properties such as SOC, TN, TP, EC, NH₄⁺-N, and ASi all decreased significantly with increasing soil depth. In the surface soil (0-10 cm), N addition significantly promoted the accumulation of POC and MAOC, with their contents reaching the maximum under the synergistic effect of N and P addition. However, single P addition inhibited the formation of POC and MAOC. In the sub-surface soil (10-20 cm), the effects of N and P addition on POC and MAOC were weaker, with MAOC exhibiting higher stability. Correlation analysis showed that the POC content was significantly negatively correlated with soil pH and significantly positively correlated with TN and NH₄⁺-N content, while soil MAOC content was significantly positively correlated with ASi. In summary, N and P addition has a significant synergistic effect on soil carbon storage in alpine grasslands. This study provides important scientific basis for the sustainable management of alpine grasslands and the enhancement of their carbon sink functions. In the future, optimizing nutrient management strategies can promote the conversion of POC to MAOC in alpine grasslands, thereby enhancing the long-term stability of soil carbon pools.

Key words: alpine grassland, soil organic carbon, particulate organic carbon (POC), mineral-associated organic carbon (MAOC), N and P addition