The soil organic carbon and its labile fractions are highest during moderate shrub expansion stage in the alpine meadow shrub expansion process

Shrub expansion has a profound influence on regional carbon cycling. However, understanding the dynamics of soil carbon fractions and their driving factors along shrub expansion gradients remains critical for accurately assessing the ecosystem's carbon sequestration potential. In this study, three different stages of Dasiphora parvifolia shrubs were identified on the eastern edge of the Qilian Mountains, with alpine meadows (non-shrub) serving as the control. We investigated the vegetation status, determined the soil carbon fractions (including total carbon, total organic carbon, microbial carbon, dissolved organic carbon, particulate organic carbon, and easily oxidizable organic carbon), and the soil microbial community in both shrub patches and the grassy matrix. We found that during shrub expansion, the herbaceous community was significantly suppressed at the LS stage (with biomass and diversity markedly reduced), partially recovered at the MS stage, and exhibited increased plant height but further declined in diversity at the HS stage, indicating stronger competition from shrub patches than from the grassy matrix. Soil carbon dynamics followed a distinct pattern: substantial carbon loss occurred at the LS stage (with TOC content decreasing by 48.6% and 31.8% compared to CK), while active carbon components (POC, DOC, EOC) significantly increased at the MS and HS stages ( P <0.05). Redundancy analysis reveals that both vegetation characteristics and microbial communities have high explanatory power for the variation in carbon fractions. Path analysis showed that in the grassy matrix, vegetation characteristics directly or indirectly influence changes in soil carbon content by affecting fungal community diversity, whereas in the shrub patch, the relationship is indirect.