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).