3.3 Niche breadths and niche overlap of phyla in degraded grasslands
To explain the responses of different phyla to grassland degradation, we further assessed the niche widths of each phylum across the different degradation levels, as well as the relationships between taxon niche width and environmental variables.
First, the mean niche width at community level in each degraded region was calculated, and then the corresponding OTUs in each phylum (top eight dominant phyla) were calculated based on the Z-score-transformed abundances of all OTUs. The niche widths of all phyla in each degraded region were obtained from the average values of corresponding Z-score-transformed OTUs, and were compared among different regions. Proteobacteria and Bacteroidetes could be distinguished from other phyla because of their broader niche widths that were significantly and negatively correlated with nutrient contents (Figure 3A, 3C and S6, P < 0.05) . On the contrary, the other major phyla (Acidobacteria, Actinobacteria, Planctomycetes, Chloroflexi, and Gemmatimonadetes) exhibited decreasing niche widths with an increase in soil nutrient contents (Figure 3A, 3C and S6, P< 0.05) . Species with broader niche breadths were distributed widely and evenly, suggesting greater stability in response to environmental disturbance (Pandit et al., 2009; Li et al., 2019). Therefore, the greater niche breadths of Proteobacteria and Bacteroidetes indicated their high adaptability and positive responses when compared to other phyla under grassland degradation.
To evaluate variation in niche overlap within each phylum across the different grassland degradation conditions, we Z-score transformed the niche overlaps of each comparison (OTU–OTU) and compared the mean values within each phylum with an increase in degree of degradation. In the two grasslands, as degradation intensified, the mean values (Z-score) of niche overlap within Acidobacteria, Actinobacteria, Planctomycetes, and Gemmatimonadetes decreased, while the average niche overlaps of Proteobacteria and Bacteroidetes increased (Figure 3B and 3D) , which may be partly due to changes in their species diversity, which would lead to greater species competition(Table 1) .
The results suggest that phyla with great survivability and others with weak survivability adopted different strategies under grassland degradation. With an increase in degree of degradation, Proteobacteria and Bacteroidetes exhibited higher alpha diversity, and niche width and niche overlap; on the contrary, other phyla (mainly Actinobacteria, Acidobacteria, Gemmatimonadetes, and Planctomycetes) exhibited decreased alpha diversity, and niche width and niche overlap (Table 1; Figure 3) , which may be attributed to the elimination of certain species with functional redundancy, which can facilitate better adaptation to grassland degradation in bacterial communities.