3.2 Responses of different phyla to grassland degradation
To decipher the responses of bacterial communities to grassland
degradation, we explored the shifts in the abundance and species
richness among the top eight phyla (relative abundance >
3%).
Proteobacteria, Bacteroidetes, and Firmicutes exhibited relatively high
species richness values, whereas other phyla, such as Acidobacteria,
Actinobacteria, Gemmatimonadetes, Planctomycetes and Chloroflexi,
exhibited relatively low richness values as degradation level increased
(1–5 in G1; 1–4 in G2) (Table 1) . The decrease in species
richness observed in the deserted regions (region 6 in G1; region 5 in
G2) could be attributed to the substantial decrease in richness at the
community level. The proportions of Proteobacteria (21.2–32.4% in G1
and 21.9–28.6% in G2) and Bacteroidetes (3.8–11.0% in G1 and
4.7–7.6% for G2) increased with an increase in grassland degradation
level (Figure 2A and 2B) , indicating that Proteobacteria and
Bacteroidetes exhibited positive responses based on biodiversity in the
course of grassland degradation.
Conversely, with advancement in degradation, bacterial communities had
more significantly altered OTUs (Figure S3) . After classifying
the altered species into corresponding phyla, we observed that the
numbers of the enriched OTUs in Proteobacteria and Bacteroidetes
increased with an increase in the degradation level in the two
grasslands (Figure 2C and 2D) . On the contrary, Actinobacteria,
Acidobacteria, Chloroflexi, and Planctomycetes had greater depleted OTUs
than other phyla with an increase in degradation level. The results
indicate that Proteobacteria and Bacteroidetes had more positive
responses with regard to survival than other phyla under grassland
degradation.
To determine the dominant bacterial biomarkers along the grassland
degradation gradient, we conducted RF to selected the top 30 most
important OTUs and classified them into phyla. Subsequently, their
correlation with environmental factors was determined using Pearson
correlation analysis. The major microbial biomarkers were classified
into the phyla Proteobacteria, Acidobacteria, and Actinobacteria in the
process of degradation (Figure S4) . In the case of
Acidobacteria, the biomarker OTUs were almost all classified into the
Subgroup_6, and were significantly and positively correlated with soil
nutrient contents (OM, TN, and TP) in G2 (Figure 2E and 2F,P < 0.05) . The Actinobacteria biomarker species in the
two grassland types were largely classified into the Thermoleophilia and
Actinobacteria classes, which were positively and negatively correlated
with nutrient content, respectively (Figure 2E and 2F) .
Furthermore, the biomarker OTUs in Proteobacteria were mostly classified
into Gammaproteobacteria in G1, and into Gammaproteobacteria and
Alphaproteobacteria in G2. The abundance of biomarkers classified into
Alphaproteobacteria were positively correlated with soil nutrient (OM,
TN, and TP) contents in G2 (Figure 2E and 2F) . However,
Gammaproteobacteria biomarker species exhibited significant and negative
correlations with nutrient contents in both grassland types(Figure 2E and 2F) .