Grazing and reclamation-induced microbiome alterations drive organic carbon stability within soil aggregates in alpine steppes

Grazing and reclamation are the major modulators of soil organic carbon (SOC) stability in alpine steppes. However, our understanding of changes in SOC caused by grazing and reclamation at the aggregate scale is limited, and the mediating role of microorganisms remains unclear. Here, based on a long-term observation transect, we selected three land-use types: a long-term enclosed natural steppe (NS), a grazed steppe (GS), and a steppe reclaimed as cropland (CL). We investigated the impacts of grazing and reclamation on soil aggregate distributions, SOC fractions and the microbial community, and microbial contributions to SOC stability. Overall, GS and CL significantly decreased the proportion of >2 mm aggregates (by 32.25–33.72%), and enhanced SOC stability within different aggregate sizes, with the highest SOC stability observed in 0.25–2 mm and <0.25 mm sizes. GS significantly decreased bacterial diversity and increased the relative abundance of Gammaproteobacteria, while CL significantly increased microbial diversity and the relative abundance of Alphaproteobacteria. Importantly, GS enhanced the complexity of the bacterial OTUs networks in >2 mm and <0.25 mm aggregates. GS and CL caused changes in the assembly processes of bacterial communities, shifting from being mainly controlled by variable selection to being mainly controlled by homogenizing dispersal. However, there were only minor differences observed in the microbial community assembly within different aggregate sizes under the same treatment. Random forest models and correlation analysis revealed that the bacterial community, especially the network patterns and community assembly, was a key determinant of SOC stability in soil aggregates. In conclusion, we emphasize the important contribution of soil bacterial network patterns and community assembly to SOC stability in the context of human activities, providing information that may be useful for developing alpine steppe management strategies to enhance soil carbon sequestration.