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.