4.3 | The roles of biotic and abiotic factors in affecting microbial responses to grazing intensity
In this experiment, grazing-induced decrease in rhizosphere bacterial richness was related to changes in 9 of 11 variables (Figure 4), consistent with a previous study in alpine meadow finding that soil microbial richness was affected by multiple biotic and abiotic factors (Zhang & Fu, 2020). Moreover, we found that all examined biotic variables were closely related to rhizosphere bacterial richness (Figure 4), highlighting the impacts of plant selection effect on rhizosphere bacterial richness (Hartmann et al., 2009; Nan et al., 2020). However, the examined variables had little effect on fungal richness in both regions (Figure 4), consistent with previous results (Yang et a., 2018). This may be due to the fact that fungal are more resistant to environmental variability relative to bacteria (Rousk & Bååth, 2011).
Previous studies have highlighted the role of soil pH in affecting microbial richness (Qu et al., 2016; Yang et al., 2018; Zhang & Fu, 2020). However, in this study, the changes in bacterial and fungal richness were not affected by soil pH in both regions. The weak effect of soil pH in our study could be explained by the results that the soil pH in this experiment was not significantly altered by grazing intensity (Table S4, S5).
Our results indicated that the bacterial and fungal community compositions in both regions were co-affected by several biotic and abiotic variables (Figure 5a). For bacteria, the rhizosphere community composition was primarily affected by plant root biomass and root carbon concentration, highlighting the importance of plant-related resources in affecting rhizosphere bacterial communities (Costa et al., 2006; Berg & Smalla, 2009). In contrast, the bacterial community composition in non-rhizosphere was primarily affected by soil available phosphorus, highlighting the role of abiotic factors in affecting non-rhizosphere bacterial composition (Tian et al., 2017; Schöps et al., 2018). Moreover, soil available phosphorus was also the primary factor affecting the fungal composition in both regions (Figure 5d,e). The important role of soil available phosphorus in affecting microbial composition may be due to the fact that the grasslands in northern China have a relatively lower soil phosphorus than other grasslands (Han et al., 2005). For example, the soil total phosphorus concentration (515 mg kg-1) in this experiment site (Table S5) was much lower than the mean value (699 mg kg-1) in grasslands of Northern America (US Geological Survey, 2001). Also, soils in this region are rich in Ca2+ and Mg2+, and phosphorus is usually bounded to these metal ions, further constraining the available phosphorus for soil fungi (Wang et al., 2021b). Importantly, the lack of soil available phosphorus was exacerbated by heavy grazing in this experiment (Table S4, S5). This is likely due to the grazing-induced strong wind erosion that the nutrient-rich, top-layer soils were removed from the grassland (Giese et al., 2013). Given that some arbuscular mycorrhizal fungi can help plants efficiently exploit and absorb soil phosphorus (van der Heijden et al., 1998), the strong relationship between soil available phosphorus and fungal community composition also suggests that some fungal taxa may play important roles in mediating the availability of soil phosphorus in this grassland. Overall, our study provides experimental evidence that grazing intensity caused changes in abiotic and biotic factors, and these changes in turn altered the soil microbial composition.