Nine-year Fertilization Promoted C Sink and Mediated Microbial Nutrient Utilization in Alpine Meadow Soil Aggregates

Increased nitrogen (N) and phosphorus (P) inputs to terrestrial ecosystems, which are caused by human activities and atmospheric deposition, significantly affect soil nutrient cycling. However, information on the variations of fertilization treatments on the nutrient utilization and carbon (C) storage mechanisms in alpine meadow soil aggregates is limited. Here, we investigated the responses of microbial nutrient utilization and C storage dynamics in soil aggregates to nine-year fertilization (N, P, and NP addition) in an alpine meadow. The results demonstrated that nine-year fertilization did not affect soil aggregate stability, however, they promoted soil organic C (SOC), total N (TN), and total P (TP) accumulation and altered the microbial nutrient-limitation patterns of soil aggregates in the alpine meadow. Compared to N and NP treatments, the P addition was optimal for improving soil aggregate-associated C storage and soil C pool management index (CMI). Furthermore, the NP treatment significantly exacerbated the microbial C limitation and microbial P limitation was significantly greater with N addition alone than with P addition alone, which was consistent with the resource allocation theory. The enhanced microbial carbon limitation caused by fertilization had no significant impact on soil aggregate-associated C storage and CMI. Simultaneously, soil nutrients and enzyme activities increased with decreasing soil aggregate size. However, these changes did not affect the microbial nutrient limitation patterns and CMI, which may have been related to the stronger nutrient protection ability of microaggregates. Therefore, nine years of fertilization increased the soil aggregate-associated C sink functions and affected soil microbial nutrient-limitation patterns by altering the soil C, N and P stoichiometry in the alpine meadow. These findings provide technical support for future alpine meadow management.