4.2 Different phyla taxa showed varied responses to grassland degradation
According to the results of the present study, environmental factors (OM, TN, and TP) significantly influenced community composition in the degraded grassland (Figure S2) . However, the low correlation between environmental factors and bacterial community composition means that environmental factors can hardly explain microbial community composition (Figure S2) ; as observed in previous studies, which highlighted the importance of stochastic processes in microbial community assembly (Ramette & Tiedje, 2007; Graham et al., 2016; Zhang et al., 2016; Zhou et al., 2017).
The relationships between microbial community assembly in the environment and ecosystem processes remain unclear (Graham et al., 2016). In the present study, we investigated bacterial community dynamics at phylum level in degraded grasslands. Previous studies have reported that different soil bacterial phyla display different responses in abundance in grasslands following shifts in nutrient conditions (Zeng et al., 2015; Dong, Shi, et al., 2021).
In the present study, although the differences in dominant herbaceous species and soil characteristics in the two grasslands may lead to different belowground bacterial community composition, similar results were observed in soil bacterial community dynamics under grassland degradation. Proteobacteria and Bacteroidetes had more positive responses to grassland degradation than other phyla, in the form of richer diversity, higher abundance, and more unique and enriched species under relatively severe grassland degradation (Figure 1 and 2; Table 1) . The increase in the abundance of unique species and species diversity (Table 1; Figure 2A and 2B) highlighted the importance of Proteobacteria and Bacteroidetes, whose taxa enhanced community function and stability under grassland degradation (Loreau & de Mazancourt, 2013; Delgado-Baquerizo et al., 2016; Wang et al., 2017). Moreover, Proteobacteria and Bacteroidetes exhibited greater abundance, richness, and broader niche-widths (Figure 1, 2C, 2D, 3A and 3B ) with an increase in degree of degradation, indicating the proportions of resources available to the two phyla may increase under degraded conditions. Conversely, the abundance of other phyla, such as Acidobacteria, Actinobacteria, Chloroflexi, and Planctomycetes, decreased with an increase in degradation. In addition, although different biomarkers were observed between the two grasslands, the biomarkers negatively correlated with nutrients content (OM, TN, and TP) were mainly assigned to Gammaproteobacteria, indicating the importance of Gammaproteobacteria in the adaptation of the community to grassland degradation. The results demonstrated that nutrient limitation in degraded grasslands has contrasting effects on different taxa.
The responses of different taxa to their habitat environments are associated with their ecological characteristics (Ho et al., 2017). Proteobacteria and Bacteroidetes are considered copiotrophic taxa, and are enriched in soil with high resource availability (Leff et al., 2015; Wang, Zhang, Liu, et al., 2021). In comparison, oligotrophic bacterial phyla, such as Acidobacteria and Chloroflexi, would outcompete copiotrophs under low resource availability (Fierer et al., 2007; Ai et al., 2015; Ho et al., 2017). However, in the present study, inconsistent results were observed, with copiotrophic phyla (Proteobacteria, Bacteroidetes) exhibiting greater survival, with higher abundance with a decrease in soil nutrient contents (Figure 1) . Previous studies have focused on stronger environmental selection associated with artificial disturbance and larger temporal and spatial scales (Cleveland et al., 2006; Fierer et al., 2012; Leff et al., 2015; Wang, Zhang, Liu, et al., 2021). However, the present study focused on the effect of grassland degradation on microbial community assembly over relatively small spatial and temporal scales, which are associated with rather mild shifts in resource availability. Moderate shifts in resource availability may offer bacterial communities with opportunities to adapt to environmental change, rather than exposing communities to strong selection by the environment. Consistently, in the present study, stochastic processes dominated community assembly, with weak selection observed, based on the observed βNTI values that ranged between -2 and +2 (Figure 4A and 4B) . Therefore, community member life strategies may be more instructive than physiological characteristics in predicting bacterial community assembly dynamics under degraded grassland conditions.