3.1. Soil properties and bacterial community composition changed with grassland degeneration
Soil nutrient contents were different among the fields in the two sampled grasslands (Table S1) . The contents of organic matter (OM; 32.31–4.44 g/kg in G1 and 20.11– 3.04 g/kg in G2), TN; 1.79–0.22 g/kg in G1 and 1.10–0.19 g/kg in G2, TP; 0.46–0.22 g/kg in G1 and 0.46–0.22 g/kg in G2) decreased with an increase in level of degradation. The highly deserted regions had the lowest OM (2.13 g/kg in G1 and 5.02 g/kg at G2), TN (0.14 g/kg in G1 and 0.30 g/kg in G2) and TP (0.18 g/kg in G1 and 0.28 g/kg in G2) contents. In addition, as the degree of degradation increased, soil moisture content decreased; soil pH did not exhibit any variation.
Bacterial richness (3526–2930 in G1 and 3709–3021 in G2) and Shannon index (9.61–8.97 in G1 and 9.72–8.36 for G2) decreased with an increase in level of degradation; furthermore, bacterial communities in the deserted regions had the lowest levels of alpha diversity(Table S2, p < 0.05) . Bacterial community composition varied significantly among regions with different degrees of degradation. Phylum Proteobacteria was the most dominant phylum in the communities in degraded grasslands (Figure 1A and 1C) . In addition, the relative abundances of phyla Bacteroidetes and Firmicutes increased with an increase in the level of degradation, and had significantly negative correlations with soil nutrient concentrations (OM, TN, and TP); however, the relative abundances of other major phyla decreased with the reduction of soil nutrient concentrations in the course of grassland degradation (Figure 1) . In addition, PERMNOVA analysis results showed significant variation in bacterial community composition among the regions with different levels of degradation (Table S3, P < 0.05 ). Bray-Curtis dissimilarity among the communities was positively correlated with OM (r = 0.398 in G1 and 0.545 in G2), TN (r = 0.388 in G1 and 0.584 in G2), TP (r = 0.460 in G1 and 0.482 in G2), and TK (r = 0.342 in G1 and 0.431 in G2) contents (Figure S2, P < 0.05) , indicating that the soil nutrient significantly influenced bacterial community composition in the degraded grassland environments.