The results of this study demonstrate that a group of biotic and abiotic variables can explain the variation in soil bacterial diversity across the four sandy grasslands in China. Specifically, our results show that the soil bacterial communities differed significantly across the sandy grasslands and were mainly controlled by a suite of variables, including edaphic variables, for instance soil EC and pH; climate variables, such as MAT and MAP; and spatial variables, namely geographic distance. Surprisingly, vegetation-related variables played a minor role in explaining the variation in bacterial community diversity.
Soil properties played an important role in explaining the variation in the diversity of bacterial communities. Soil EC was the most important factor regulating diversity patterns across a large scale. This is surprising because saline soils are often controlled by EC. In this study, both bacterial phylotype richness and phylogenetic diversity decreased with increasing EC (Figure S2); however, EC was the third most important variable for β diversity. These results are in agreement with other results obtained in saline environments27–29. The role of soil EC in determining microbial diversity can be explained by examining the variations in the relative abundances of specific taxonomic groups, especially the dominant groups. For example, in this study, the relative abundance of Actinobacteria decreased with increased EC, but the relative abundance of Proteobacteria increased (Table S3). Proteobacteria and Actinobacteria accounted for 66.56% of the obtained bacterial sequences. This indicates that salinity is highly influential not only in saline soils but also in nonsaline soils.
Soil EC is not the only stressor for soil biota in desert environments and is often accompanied by high pH30. Our results indicate that soil pH was another important factor influencing the structure of soil bacterial communities at a large scale. The contribution of soil pH in regulating the structure of soil bacterial communities has also been widely estimated by previous studies5,31−33. The strong relationship between soil pH and the structure of microbial communities can be attributed to the interaction between pH and other environmental variables. The intracellular pH of most microorganisms is usually within 1 pH unit of neutral34. Any significant variation in environmental pH imposes stress on single-celled organisms, which has significant effects on microbial community composition in a variety of environments35–37.
In addition, our findings demonstrate that soil nutrient contents, such as C,N and P, were not strong predictors of soil bacterial α and β diversity, which is consistent with the findings of a study conducted in the Loess Plateau by Zeng et al.11. However, this is inconsistent with the global soil bacterial diversity patterns identified in a meta-analysis of 600 soil samples13. This inconsistency arose because in the current study, there were no obvious differences in soil C, N and P levels across the four sandy grasslands. Thus, no significant correlations were observed between soil bacterial α and β diversity and soil C, N and P contents. However, multicollinearities were excluded from the step wise regression, and only variables that were closely related, such as soil EC, were selected, as they contributed to most of the variance.
Vegetation properties (PR and AB) exhibited weak but significant correlations with bacterial phylotype richness and phylogenetic diversity but not with bacterial community structure (Table 3; Table S1). Many studies have examined the relationships between vegetation characteristics and soil bacterial community properties; however, the results have been inconsistent. Some studies in grassland ecosystems have reported the significant effect of plant diversity on the diversity of soil microbial communities38,39. Others have shown that vegetation properties do not significantly affect the soil microbial community40. Plant diversity and productivity control the contents of limiting resources and thereby control the soil biota41,42. The results of this study confirm the importance of resource availability in regulating the soil microbial community in these sandy grasslands and indicate that the influences of vegetation properties on soil biodiversity in sandy grassland ecosystems should also be considered in future studies.
Climate also played an important role in explaining the variation in the structure of soil bacterial communities. Our findings reveal that MAT and MAP significantly affected the structure of soil bacterial communities. This finding is consistent with studies demonstrating the importance of MAT and MAP in influential the structure of bacterial communities at different scales2,43, especially in arid and semiarid regions such as temperate grasslands in northern China44. Any change in MAT or MAP directly affects the processes of microbial metabolism45. Furthermore, MAT and MAP are the main factors affecting plant community composition and AB, which determine the quantity and quality of resources available to soil bacterial communities5,46,47. In this sense, MAT and MAP control the structure of soil bacterial communities through their impacts on vegetation properties and soil nutrient levels.
The strong relationship between the degree of dissimilarity among bacterial communities and geographical distance indicates that the probability of dispersion is the main constrainton the spatial patterns of bacterial communities across the four sandy grasslands. The impacts of dispersal limitation in soil bacterial communities have also been extensively described in previous studies31,48,49. However, the results of this study are inconsistent with the results of Fierer and Jackson2, who quantitatively compared the diversity and structure of bacterial communities across North and South America, and found that there was no significant relationship between the degree of dissimilarity among soil bacterial communities and geographic distance. This discrepancy may be attributed to the different scales of that study and our study50,51. Our results are in agreement with those of Martiny et al.52, who suggest that the effects of dispersal limitation on the biogeography of soil bacterial communities are most likely to be observed at intermediate scales (10–3,000 km).
In summary, this work is the first attempt to comprehensively detect the geographic patterns of soil bacteria across the four sandy grasslands in northern China. A combination of biotic and abiotic factors controlling soil bacterial α and β diversity were identified. Our study provides evidence for the influence of soil EC on the diversity of bacterial communities, which is often neglected in nonsaline soils. Soil pH was another important predictor of the structure of soil microbial communities. Geographic distance and climate factors (MAT and MAP) explained the largest proportion of the variance in the structure of soil microbial communities. To improve our knowledge about the dynamics of these sensitive and fragile ecosystems, future efforts should be put into exploring the general patterns that microbes respond to climate change and human activities. The results of this study strengthen our knowledge about the processes that generate and maintain microbial biogeographic patterns.