Degradation-driven bacterial homogenization closely associated with the loss of soil multifunctionality in alpine meadows

Alpine meadows are sensitive to climate change due to their unique ecosystem. The mechanisms by which soil microbial diversity regulates soil multifunctionality under degradation need to be extensively explored. In this study, three degraded levels of alpine meadows (nondegraded, ND; moderately degraded, MD; and severely degraded, SD) were chosen in the hinterland of Sanjiangyuan National Park on the Qinghai–Tibet Plateau. Soil microbial abundance and community structure were determined through 16 S rRNA, and six soil variables related to nutrient pools (DNA concentration, CDNA; soil organic matter, SOM; total N, TN; total P, TP; hydrolytic N, HN; and available P, AP) were measured and calculated for soil multifunctionality. The results showed that meadow degradation resulted in a decline in bacterial functional diversity and an increase in compositional diversity (Shannon index and richness). Soil fungi were less sensitive to meadow degradation. Meanwhile, soil individual functions, such as SOM, TN, HN, AP, and multifunctionality, decreased significantly when subjected to meadow degradation. Moreover, bacterial functional diversity and the Shannon index, rather than bacterial richness, had significantly positive and negative relationships with soil multifunctionality, respectively. Furthermore, the dissimilarity of bacterial communities decreased with degradation. These results indicated that meadow degradation led to bacterial homogenization and functional redundancy, all of which would reduce soil multifunctionality. Abiotic factors such as soil water content (SWC) and bulk density (BD) also significantly changed and still played a key role in regulating the soil multifunctionality accompanying degradation. Amino-acid metabolism had the highest relative abundance (4.37%) and was negatively correlated with SOM and TN, implying that bacteria may convert more limited resources for their own needs and increase the loss of soil multifunctionality under severe degradation. The results provide new insight into the relationships between biodiversity and ecosystem multifunctionality and scientific guidance for the restoration of degraded alpine meadows.