Increased chemical stability but decreased physical protection of soil organic carbon in response to nutrient amendment in a Tibetan alpine meadow

Nutrient amendment increases plant productivity but the effects and mechanisms on soil organic carbon (SOC) accumulation and stability remain unclear, especially in nutrient deficient alpine ecosystem. Here, based on an experiment combining nitrogen (N) and phosphorus (P) input continuously for 15 years, we found that nutrient amendment did not affect total SOC content, but increased mineral-associated C with decreasing soil aggregate stability. Despite increased total phospholipid fatty acid (PLFA) and bacteria PLFA, nutrient amendment decreased soil enzyme activities involved in C cycling. The 13C NMR analyses showed that nutrient amendment decreased the aliphaticity but enhanced aromaticity of SOC. Structural equation models indicated that P availability (Olsen-P content) was most related to shifts in microbial community composition and decreased enzyme activities. Moreover, the concomitantly reduced aggregation and increased mineral-associated C were mainly attributable to the decrease of fungal biomass and increase of bacterial biomass. Together, interconnected factors such as increased acidity, aggregate destabilization, microbial community shift towards bacteria, and loss of oxidative enzyme activities could contribute to the overall response of SOC under intensive N and P input. In particular, available P rather than N may re-shape the pattern of physical and chemical stabilization of SOC, shifting from moderately physical protection to highly chemical stability, implicating the pivotal roles of P management in C cycling of alpine ecosystem.