Soil bacterial responses to salinity
The soil salt, EC, and BD values significantly increased, while significantly lower values of TC were observed as the salinization level increased (Table S1; p < 0.05). The α-diversity of soil bacteria had a consistent response to soil salinity. S2 showed higher Sobs, ACE, and Chao 1 indices than CK, S1, S3, and S4. Significantly lower values of Sobs, Shannon, ACE, and Chao 1 indices were observed in S4 (Table 1; p < 0.05).
The NMDS and ANOSIM tests showed that the bacterial community composition in S2, S3, and S4 differed significantly from that in the CK at the OTU level (stress = 0.0001, R = 0.9596, p = 0.001; Fig. S1a). In addition, the NMDS and ANOSIM tests showed that the bacterial community composition in S1, S2, S3, and S4 differed significantly from that in the CK at the phylum level (stress = 0.09, R = 0.92, p = 0.001; Fig. 1a) and the class level (stress = 0.05, R = 0.90, p = 0.001; Fig. S2a), and the Shannon diversity at the phylum level in S1, S2, S3, and S4 also differed significantly from that in the CK (stress = 0.08, R = 0.79, p = 0.001; Fig. 1b). The phylum Proteobacteria occupied the largest proportion across the five salinity levels. Soil salinization significantly increased the relative abundances of Gemmatimonadetes, and S3 plot have the highest abundances of Bacteroidetes (Fig. 2a). However, the relative abundances of Actinobacteria and Acidobacteria under high salinity were lower than those under low salinity (Fig. 2a). Furthermore, the class Alphaproteobacteria and Betaproteobacteria under high salinity were lower than those under low salinity, and soil salinization significantly increased the relative abundances of Gammaproteobacteria (Fig. S2b).
The NMDS ordinations based on the relative abundance of the bacterial communities (a) and Shannon diversity (b) of the soil bacterial phyla under five salinity levels.
The relative abundances (a) and Shannon diversity (b) of the soil bacterial phyla under five salinity levels; significant relationships at p < 0.05 are indicated by different letters based on the DUNCAN test.
The Shannon diversity at the phylum level is shown in Fig. 2b. Soil salinization significantly increased the Shannon diversity of Bacteroidetes. However, the Shannon diversity of Actinobacteria and Chloroflexi decreased dramatically at extremely high salinity (S4). Interestingly, the Shannon diversity of Proteobacteria, Acidobacteria, Gemmatimonadetes and Cyanobacteria showed an increase from CK to S3 and then a decrease as the salt increased from S3 to S4.
Soil properties structuring the bacterial communities and diversity
The C/N ratio (r = 0.367, p = 0.003) and soil EC (r = 0.91, p = 0.001) were positively correlated with soil salt (r = 0.821, p = 0.001), which significantly affected the bacterial communities at the OTU level based on the Mantel test (Fig. S1b). The C/N ratio (r = 0.594, p = 0.001) and soil EC (r = 0.742, p = 0.001) were positively correlated with soil salt (r = 0.563, p = 0.001), which significantly affected the bacterial communities at the phylum level based on the Mantel test (Fig. 3a). At the class level, we also found that the soil salt (r = 0.718, p = 0.001), EC (r = 0.869, p = 0.001), C (r = 0.601, p = 0.001), N (r = 0.658, p = 0.001), and C/N ratio (r = 0.547, p = 0.001) significantly affected the bacterial communities (Fig. S2c). In addition, soil EC (r = 0.739, p = 0.001), soil salt (r = 0.699, p = 0.001), C (r = 0.598, p = 0.001), N (r = 0.617, p = 0.001), and the C/N ratio (r = 0.293, p = 0.014) significantly influenced the microbial Shannon diversity (Fig. 3b).
The RDA plots showing the effects of soil physiochemical properties (pH, EC, salt content, TC, TN, and C/N ratio) on the bacterial community structure at phylum level (a) and Shannon diversity (b). The significance of the effect of each property was examined using the Mantel test (permutation = 999), and the significance was evaluated by the r- and p-values.
Spearman correlation analyses showed that soil pH was not correlated with the bacterial abundance and diversity (Fig. 4). The relative abundances of Acidobacteria, Actinobacteria, Nitrospirae, and Cyanobacteria were negatively correlated with soil salt, EC, and the C/N ratio and positively correlated with soil C and N. In addition, The relative abundances of Alphaproteobacteria and Betaproteobacteria were negatively correlated with soil salt, EC, and the C/N ratio and positively correlated with soil C (Fig. S2d). However, the relative abundances of Firmicutes, Gemmatimonadetes, and Bacteroidetes were positively correlated with soil salt, EC, and the C/N ratio and negatively correlated with soil C and N. Interestingly, only the Bacteroidetes Shannon diversity showed a positive correlation with soil salt, EC, and the C/N ratio compared with the other phyla (Fig. 4).
Spearman correlation analyses between soil physiochemical properties and the relative abundances and Shannon diversity of soil bacterial phyla. EC: electrical conductivity, TC: total carbon, TN: total nitrogen, and C/N ratio: soil total carbon/nitrogen. *, **, and *** indicate significance along the paths at p < 0.05, p < 0.01, and p < 0.001 levels, respectively.
Source: Ecology - nature.com
