4.1 Stochastic processes dominated community assembly under natural grassland degradation
Both deterministic and stochastic community assembly processes play important roles in shaping community composition and structure in natural ecological systems (Chase, 2010; Chase & Myers, 2011; Zhou et al., 2013). Therefore, elucidating the assembly processes of microbial community could enhance the understanding of the responses of communities to grassland degradation. However, numerous studies that emphasized the importance of environmental filtration have focused on determining the dominant environmental factors driving and structuring bacterial community structure (Feng et al., 2017; Knelman et al., 2019; Yu, Wu, et al., 2019; Liu, Graham, et al., 2020), with the assembly processes in bacterial communities in the course of grassland degradation and the community response strategies remaining poorly understood. Therefore, in the present study, the authors investigated the turnover and assembly strategies of bacterial communities in response to shifts in resource conditions under grassland degradation. Overall, the authors observed that bacterial community assembly processes in degraded grassland systems were dominated by stochastic processes (Figure 4A and 4B) .
Numerous researchers have reported that as environmental change is prolonged, deterministic processes may become more important due to the accumulation of environmental selection; however, the above often occurs after stochastic processes have dominated the initial phase (Ferrenberg et al., 2013; Zhang et al., 2016; Liu, Graham, et al., 2020). Therefore, the stochastic processes observed in the present study could be associated with the short term and limited spatial scales applied for the sampled grasslands under degradation, and continuous environmental change could alter the dominant community assembly processes(Figure 4A and 4B) . In addition, degree of change in environmental variables is a key factor influencing community assembly process at the regional scale (Feng et al., 2018). In the present study on a degraded grassland with low human interference, the mild changes in environmental variables could be another major reason for the dominance of stochastic processes in community assembly processes (Liu, Graham, et al., 2020).
Stochastic processes represent undetermined ecological assembly processes that yield community diversity patterns that are random (Zhou & Ning, 2017). Stochastic processes (e.g., colonization and extinction) are as important as or more important than deterministic processes (e.g., species selection and species interactions) in structuring natural microbial community (Chase & Myers, 2011).Although numerous studies have highlighted the importance of stochastic factors in driving the structure and functions of communities (Zhou et al., 2008; Caruso et al., 2011; Chase & Myers, 2011; Stegen et al., 2012; Zhou et al., 2013; Zhou & Ning, 2017), the dominance of stochastic process in community assembly makes it extremely challenging to explore the underlying community dynamics because of the unpredictable compositional variation (Zhou et al., 2013; Zhang et al., 2016). Consequently, we investigated community composition dynamics in grasslands with different levels of degradation in an attempt to elucidate the corresponding mechanisms of bacterial community assembly.