4.2 | Responses of bacterial and fungal communities to grazing intensity
An intriguing result from our experiment was that the rhizosphere bacterial richness was more sensitive to grazing intensity than that in non-rhizosphere (Zhang et al., 2019). As indicated in Figure 2, the rhizosphere bacterial richness sharply declined under heavy grazing intensity, whereas that in non-rhizosphere remained unchanged (Zhang et al., 2020). Two mechanisms may underlie grazing-induced decline in rhizosphere bacterial richness. First, herbivory removes aboveground organs of a plant, resulting in a reduction in assimilated carbon allocated to belowground organs, and thus a decline in the quantity of carbon sources available for bacteria in plant rhizosphere (Aldezabal et al., 2015; Mueller et al., 2017; Byrnes et al., 2018), which in turn reduces rhizosphere bacterial richness. In this study, the rhizosphere bacterial richness was significantly affected by root carbon (Figure 4), providing support for this mechanism. Second, plant diversity is usually coupled with microbial diversity in a community (De Deyn & van der Putten, 2005). Thus, grazing-induced reduction in plant diversity may decrease the diversity of carbon sources for bacterial taxa (Dwivedi et al., 2017), as a consequence, some bacterial taxa cannot grow well or disappear and the rhizosphere bacterial diversity declines.
In contrast to the response of rhizosphere bacterial richness, the fungal richness was not significantly affected by grazing intensity in both regions (Figure 2c,d), which is consistent with results from grazing experiments (Zhang & Fu, 2020; Yang et al., 2021). The weak response of fungal richness may be due to the fact that fungi were more tolerant to environmental stresses than bacteria (Rousk & Bååth, 2011). For example, fungi often exhibit more resistance to soil drought or acidity than bacteria (Rousk & Bååth, 2011). In this study, the fungal richness was not significantly affected by examined biotic and abiotic factors (Figure 4), implying that grazing-induced changes in these variables did not exert significant impact on fungal richness. Overall, our results suggest that grazing in this grassland has little impact on soil fungal richness.
For bacterial composition, previous studies have almost exclusively concentrated on non-rhizosphere bacteria (Hu et al., 2017; Zhang et al., 2020; Wang et al., 2021a). Our results suggest that grazing intensity has stronger impacts on bacterial composition in rhizosphere than that in non-rhizosphere. For example, heavy grazing resulted in a 6.55-fold increase in community dissimilarity in rhizosphere while a 1.89-fold increase in non-rhizosphere (Figure 3a). An impressive result from this experiment was that, with the increase of grazing intensity, the relative abundance of a dominant bacterial phylum,Proteobacteria , decreased in non-rhizosphere but increased in rhizosphere (Figure 3b, c). Such opposite responses to grazing intensity were likely due to its copiotrophic strategy in nutrient use (Leff et al., 2015). Increased grazing intensity led to a relatively higher plant root nutrient (Tables S4, S5), which facilitates the growth of copiotrophic bacteria. This is supported by our results that the relative abundance of Proteobacteria in rhizosphere was positively related to plant root nitrogen (Figure S1a). By contrast, grazing-induced decrease in the relative abundance ofProteobacteria in non-rhizosphere was related to the reduction in soil total nitrogen (Figure S1b; Tables S4, S5).
Our results also demonstrated strong impacts of grazing intensity on fungal composition in this grassland. As indicated by the increased dissimilarity of fungal community with grazing intensities (Figure 3d), the fungal composition was dramatically altered by heavy grazing in both regions. A significant increase in relative abundance was observed for a dominant taxon, Ascomycota , under heavy grazing intensity (Figure 3e,f). This phenomenon could be explained by its oligotrophic strategy, which facilitates its growth in relatively low carbon environment (Sterkenburg et al., 2015). As indicated in this experiment, heavy grazing resulted in a great reduction in carbon concentrations in plant roots and soils (Tables S4, S5), and the relative abundance ofAscomycota was negatively related to these carbon concentrations (Figure S2a, b).