Anthropogenic activities and atmospheric deposition have increased the nitrogen (N) and phosphorus (P) inputs to terrestrial ecosystems, which can significantly alter ecosystem carbon cycling. To better understand the mechanisms of soil organic carbon (SOC) responding to nutrient fertilization, we measured physical fractions (by particle-size fractionation) and chemical composition (by solid-state ¹³C NMR spectroscopy) of SOC, plant biomass and nutrient concentration, soil chemistry, microbial biomass and community composition after 10 years of N and P addition in an alpine meadow on the Tibetan Plateau. Our results showed that total SOC and mineral-associated organic carbon (MAOC) contents were not affected by N and P addition. However, P addition promoted particulate organic carbon (POC), which was likely attributed to hampered decomposition by lower microbial biomass (particularly fungi). In contrast, N addition did not change POC, probably because more plant biomass inputs were offset by faster decomposition of higher-quality plant litter (lower C:N ratio). Moreover, N addition rather than P addition decreased the percentage of labile functional group of O-alkyl C, whereas slightly increased alkyl-aromatic C:O-alkyl C ratio. These changes in SOC chemical composition with N inputs were likely caused by enhanced labile OM decomposition and rhizodeposit inputs. Overall, our results suggest that long-term exogenous N input could potentially accelerate SOM decomposition indicated by the chemical composition, but P input could result in inhibition of SOM decomposition and accumulation of POC stock in the alpine meadow ecosystem.

人为活动和大气沉积增加了陆地生态系统的氮（N）和磷（P）输入，这可以显着改变生态系统的碳循环。为了更好地理解土壤有机碳（SOC）对养分施肥的响应机制，我们测量了物理分数（通过粒度分级）和化学成分（通过固态^13）核磁共振波谱分析）在青藏高原高寒草甸中添加氮和磷10年后，土壤有机碳，植物生物量和养分浓度，土壤化学，微生物生物量和群落组成。我们的结果表明，总的SOC和与矿物相关的有机碳（MAOC）的含量不受氮和磷的添加的影响。然而，磷的添加促进了颗粒有机碳（POC）的产生，这很可能归因于较低的微生物生物量（尤其是真菌）阻碍了分解。相反，氮的添加不会改变POC，这可能是因为更高质量的植物凋落物的更快分解（较低的C：N比）抵消了更多的植物生物量输入。而且，N添加而不是P添加降低了O-烷基C的不稳定官能团的百分比，而烷基-芳族C：O-烷基C的比率略微增加。带有N输入的SOC化学成分的这些变化可能是由于不稳定的OM分解和根状沉积输入的增强引起的。总体而言，我们的结果表明，长期的外源氮输入可能潜在地加速化学成分所指示的SOM分解，但是P输入可能导致SOM分解和高寒草甸生态系统中POC蓄积的抑制。