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?