1 | INTRODUCTION
Grasslands cover about 40% of Earth’s land surface and are mostly used
for livestock grazing (Suttie, Reynolds, & Batello, 2005). While light
and/or moderate grazing can improve the diversity and productivity of
grasslands, heavy grazing usually causes severe grassland degradation
(Kemp et al., 2013; McNaughton, 1979). By far, overgrazing-induced
grassland degradation has become a major ecological problem worldwide
(Dlamini, Chivenge, & Chaplot, 2016; Eldridge et al., 2017; van de
Koppel, Rietkerk, & Weissing, 1997). This is especially the case in
China, where more than 90% of grasslands are in a degraded status (Kemp
et al., 2013).
Soil microbes play important roles in regulating grassland structure and
functioning (Bardgett & van der Putten, 2014). However, soil microbial
communities are prone to be affected by heavy grazing (Hu et al., 2017;
Lame, Kennedy, & Siciliano, 2011; Wang et al., 2021a; Zhang et al.,
2020). Grazing can influence the soil microbe through several
mechanisms.
First, herbivory on plants can have dual effects on soil microbial
communities. On the one hand,
removal
of above-ground biomass decreases the litter input into the soil, which
reduces the supply of carbon resource to soil microbe,
and
thus causes reduction in soil microbial richness (Dwivedi et al., 2017).
On the other hand, removal of above-ground parts may also stimulate the
allocation of organic nutrients to belowground organs (Mueller et al.,
2017), resulting in enrichment of root
exudates
(Hamilton & Frank, 2001), which in turn affects the structure of
microbial community (Hu et al., 2018). Second, manure from livestock may
affect soil pH values (Wang et al., 2021a; Yang et al., 2018) and
improve the available soil nitrogen (Han et al., 2008). As a result,
the
relative abundance of
copiotrophic
taxa were enhanced while that of oligotrophic taxa was reduced (Nie et
al., 2018). Finally, livestock trampling decreases soil moisture due to
increased soil compactness (Kobayashi, Hori, & Nomoto, 1997), which in
turn affects the soil microbial properties (Chen et al., 2019).
These
studies provide important insights into grazing impacts on soil
microbial communities, however, most studies did not distinguish the
microbial responses between non-rhizosphere and rhizosphere regions.
Given that plant rhizosphere is a micro-region that plant-soil-microbe
interacts directly (Curl & Truelove, 1986), the influence of grazing on
microbial communities in this region is likely stronger than those in
non-rhizosphere.
Here we examined the impacts of grazing intensity on the richness and
composition of microbial communities in rhizosphere vs. non-rhizosphere
by a 15-year grazing intensity experiment in a semiarid grassland. We
focused on three specific questions: i) How do microbial communities
differ in richness and composition between rhizosphere and
non-rhizosphere? ii) How do bacterial and fungal communities respond to
grazing intensities? iii) What are the relative roles of abiotic and
biotic factors in affecting bacterial and fungal responses to grazing
intensity?