3.4 Assembly processes and phylogenetic structuring under grassland degradation
The βNTI values of the two grasslands were mainly between -2 and +2(Figure 4A and 4B) , indicating the dominance of stochasticity and weak environmental selection during community assembly across different levels of grassland degradation.
According to the results of correlation analysis, Proteobacteria and Bacteroidetes relative abundance had no significant correlation with phylogenetic clustering; furthermore, there were no significant correlations between environmental variables and ses.MNTD(Figure 4C and 4D, P > 0.05) . However, soil nutrient contents were negatively correlated with ses.MNTD values in other phyla (Figure 4C and 4D, P < 0.05) , indicating that soil nutrient loss decreased phylogenetic clustering in bacterial communities. Considering the decrease in the niche overlap(Figure 3B and 3D) and the correlation between alpha diversity and ses.MNTD of the phyla across different levels of grassland degradation (Table 2) , the phylogenetic dispersion could have resulted from the elimination of some close phylogenetic species with functional redundancy.