Soil bacterial communities vary with grassland degradation in the Qinghai Lake watershed

Aims: Grasslands are experiencing severe degradation globally, impacting aboveground vegetation and soil properties. The influences of grassland degradation on bacterial communities in soil are not well-understood. Methods: The normalized difference vegetation index (NDVI) was calculated to represent grassland status and indicate grassland degradation (decreasing NDVI). Soil ph, bacterial communities, as well as nutrient and organic carbon concentrations were measured. Results: Bacterial alpha diversity had negative relationships with soil moisture, soil organic carbon (SOC), total nitrogen (TN), and total phosphorus (TP). Bacterial community structure was significantly associated with NDVI, the change rate of NDVI, moisture, ph, SOC, TN, as well as soil C:N and C:P ratios. Bacterial phyla were differentially related with these environmental variables. Moreover, network analysis showed that the network of soil bacteria had strong cooperation relationships (positive correlations between taxa) and was grouped into three modules. According to modularity, 71 keystone taxa were detected as network connectors and module hubs. All the modules had close relationships with environmental variables, and many keystone taxa were negatively associated with soil moisture and SOC. Conclusions: These results suggest that grassland degradation might be responsible for shifts in soil bacterial communities in terms of alpha diversity and community structure through changes in soil moisture, SOC, nutrients, and C:nutrient ratios. Moreover, in the network analyses, the strong co-occurrence relationships between taxa as well as close relationships between environmental variables and module structures and keystone taxa suggest low stability and high vulnerability of bacterial communities to the influences of grassland degradation.