Sensitivity of soil carbon dynamics to nitrogen and phosphorus enrichment in an alpine meadow

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.