Microbial community assembly mediated by soil organic carbon stability in aggregates under long-term enclosure alpine meadows

Enclosure is a key strategy for restoring degraded grasslands and increasing soil organic carbon (SOC) storage. However, the microbial mechanisms underlying SOC stability at the soil aggregate scale remain unclear. We used a 37-year long-term enclosure experiment in alpine meadows in the semi-arid region of Central Asia to explore the effects of enclosure on the soil organic carbon fractions and microbial communities. The findings revealed that enclosure drove the functional differentiation of SOC within aggregates, significantly increasing very labile SOC (F1), labile SOC (F2), and less labile SOC (F3) contents, while decreasing oxidizable resistant SOC (F4) contents. Enclosure shifted microbial life strategies from r- to K-selection, reducing the fungal-to-bacterial ratio and enhancing fungal co-occurrence networks. Bacterial community assembly was governed by deterministic processes, particularly homogeneous selection, whereas fungal communities were shaped by stochastic processes, resulting in a group-specific assembly pattern described as “bacterial determinism-fungal stochasticity”. Partial least squares path modeling revealed that microbial assembly processes and network properties jointly influence SOC stability at the aggregate scale, with keystone taxa mediating their interactions. Overall, enclosure increased SOC sequestration capacity by reorganizing the spatial distribution of SOC fractions within aggregate microhabitats, and modulating microbial life strategies, assembly processes, and cross-kingdom interactions. Thus, the shifts in SOC fractions and microbial community assembly patterns serve as ecological indicators for assessing restoration efficacy, and enclosure is an effective management practice for restoring degraded grassland.