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Figure 1 . Bacterial community composition at phylum level across different degradation levels in grassland 1 (A ) and grassland 2 (C ); Pearson correlation between relative abundance of dominant phyla and soil nutrient contents in grassland 1 (B ) and grassland 2 (D ).
There were six levels of degradation in grassland 1 and five levels of degradation in grassland 2, which are represented by numerical indices. The magnitude of the number represents the level of degradation. A smaller numerical index indicates a lower degradation level.
The red blocks in heatmap represent positive correlation and the blue blocks represent negative correlation. “*”, “**”, “***” represent significant correlations with P values less than 0.05, 0.01, and 0.001, respectively. TP, total phosphorous; TN, total nitrogen; TK, total potassium; OM, organic matter.
Figure 2 . Bacterial community dynamics in response to grassland degradation. The proportions of unique species of different phyla in communities across the degradation levels in grassland 1 (A ) and grassland 2 (B ); The proportions of enriched and deleted species of different phyla in communities across different degradation levels in grassland 1 (C ) and grassland 2 (D ); Heatmap showing correlations between the relative abundances of the top 30 predictive operational taxonomic units (OTUs) and nutrient contents in grassland 1 (E ) and grassland 2 (F ).
The magnitude of the number represents level of degradation. A smaller numerical index indicates lower degradation level.
The enriched (depleted) OTUs indicated that the relative abundance of OTUs in degraded grassland regions was significantly higher (lower) than that in the region with the least degradation level.
The red blocks in heatmap represent positive correlation and the blue blocks represent negative correlation. “*”, “**”, “***” represent significant correlations with P values less than 0.05, 0.01, and 0.001, respectively.
Figure 3 . Variation in niche breadth and niche overlap of phyla under grassland degradation. The mean niche breadths (Z-score-transformed) of members in each phyla across the degradation levels in grassland 1 (A ) and grassland 2 (B ); The mean niche overlaps (Z-score-transformed) of members in each phyla across the different degradation levels in grassland 1 (C ) and grassland 2 (D )
Figure 4 . Assembly patterns of bacterial communities under grassland degradation. Patterns of between-community nearest taxon index (βNTI) across different degradation levels in grassland 1 (A ) and grassland 2 (B ). Horizontal dashed lines indicate upper and lower significant thresholds at +2 and -2, respectively; The correlations between ses.MNTD values of phyla and synergic soil nutrient content (Z-score-transformed) in grassland 1 (C ) and grassland 2 (D ), respectively. PB indicates community members belonging to Proteobacteria and Bacteroidetes, OUT-PB indicates all community members excluding Proteobacteria and Bacteroidetes.
Figure 5 . Correction between the average amounts of KO genes within each species and ses.MNTD values of communities.
Figure 6. Conceptual model of response strategies of bacterial communities under grassland degradation. In the conceptual model, stochasticity dominates the assembly processes of bacterial communities in response to grassland degradation with mild/continuous resource changes. Along the degradation gradient, the members (mainly Proteobacteria and Bacteroidetes) with r - strategy gradually increase to improve community productivity and opportunity, while other members with K - strategy (mainly Acidobacteria, Actinobacteria, and Chloroflexi) decrease due to adjustment due functional redundancy for enhanced community efficiency. A balance in community betweenr - and K - strategies facilitates adaptation to resource disturbance under grassland degradation.