1. Introduction
Grasslands are rich natural resources and cover approximately 40% of the global land surface (Hu et al., 2016). Grassland host biodiversity, and participate in climate regulation, carbon (C) sequestration, water purification, and erosion control, among other functions, which are critical ecosystem services and are essential for ecological stability (Habel et al., 2013; Bengtsson et al., 2019; Lyu et al., 2020). However, due to climate change and human activity, almost a half of the grassland ecosystems are undergoing degradation (Gang et al., 2014), with subsequent biodiversity loss and ecosystem function impairment (Han et al., 2020).
Soil microbial communities regulate soil biogeochemistry and ecological function (Morin & McGrady-Steed, 2004; Yang et al., 2013). Researchers have previously attempted to elucidate the responses of microbial communities to grassland degradation, which could facilitate the restoration of degraded grassland ecosystems (Yao et al., 2018; Han et al., 2020; Raiesi & Salek-Gilani, 2020).
The diverse micro habitats and available nutrient resources in grasslands potentially influence soil microbial community structure (Maharning et al., 2009). In addition, shifts in vegetation composition and soil characteristics in the course of grassland degradation can influence soil microbial community structure and activity (Bardgett et al., 2001; Yao et al., 2018; Han et al., 2020). Plant growth and development are highly correlated with soil factors, so that plant activities can indirectly influence soil microbial community structure through their influence on soil factors (Ke et al., 2015; Chen et al., 2016; Yao et al., 2018; Widdig et al., 2020). Furthermore, soil nutrient conditions directly influence soil microbial community structure (Yang et al., 2013; Wang, Wang, et al., 2018, Widdig et al., 2020). As elements essential for microbial metabolism, C and nitrogen (N) can regulate soil microbial community diversity and composition based on their contents in soil (Siciliano et al., 2014; Delgado-Baquerizo et al., 2016; Liu, Jiang, et al., 2020; Widdig et al., 2020). Soil C content can influence microbial biomass and indirectly influence microbial diversity, which is a major mediator of the relationship between soil microbial diversity and biomass across different biomes (Bastida et al., 2021). N accumulation in soil can cause bacterial diversity loss since some microorganisms are not adapted to nutrient-rich and acidic environments (Nie et al., 2018; Wang, Liu, et., 2018; Liu, Jiang, et al., 2020). Additionally, soil pH could influence bacterial community diversity and composition (Zhalnina et al., 2015; Ren et al., 2018; Tan et al., 2020; Widdig et al., 2020), nutrient solubility and availability (Zhalnina et al., 2015), and bacterial interactions and biological activity (Rashid et al., 2014; Fan et al., 2018), in addition to regulating soil microbial community assembly processes (Fan et al., 2018; Tan et al., 2020; Widdig et al., 2020).
Previous studies that have investigated the influence of soil nutrient loss on soil microbial community structure under grassland degradation have concluded that soil nutrients regulate microbial biomass, diversity, and community composition in grasslands (Hu et al., 2014; Dong, Shi, et al., 2021); however, it is not clear how microbial community assembly in turn respond to grassland degradation. Some studies have suggested that the reason for the microbial community dynamics in degraded grasslands are associated with different responses of different phyla to soil nutrient loss and decrease in available substrate (Delgado-Baquerizo et al., 2016; Dong, Shi, et al., 2021); furthermore, biotic interactions such as facilitation, niche complementation, and competition, as major drivers of community assembly (Dong et al., 2019), influence the responses of microbial communities to modulate resource-use efficiency (Yu, Polz, et al., 2019).
Distinct life strategies also help microbial communities to better respond to changes in environmental conditions (Ho et al., 2017; Vadstein et al., 2018; Li et al., 2021; Wang, Zhang, Li, et al., 2021). The balance between r -selection and K -selection strategies in a community determines the productivity levels of communities and the survival of individuals (Pianka, 1970; Reznick et al., 2002; Ye et al., 2018). In addition, soil microorganisms in arid ecosystems tend to host oligotrophic communities rather than copiotrophic communities, highlighting the potential of oligotrophic microbial communities to serve as rich sources of novel functions under resource scarcity (Chen et al., 2021). In contrast, the accumulation of soil nutrients such as N promotes a more active copiotrophic community as a result of the shift in microbial phylogenetic, metagenomic and catabolic responses (Fierer et al., 2012). Such studies highlight the importance of microbial life strategies in facilitating responses to environmental change. Therefore, considering the underlying mechanisms via which microbial communities respond to grassland degradation remain poorly understood (Luo et al., 2020; Dong, Shi, et al., 2021; Ren et al., 2021), more studies should explore the response strategies of communities and the dominant community assembly processes in the course of grassland degradation to elucidate the responses of microbial communities across environments with different levels of degradation (Yang et al., 2013). The results of such studies could facilitate further studies as well as grassland ecosystem management activities.
In the present study, we collected soil samples from two grasslands with different degrees of degradation and analyzed the variation in community diversity, composition, and function, in addition to niche breadth and phylogenic turnover among different phyla across resource gradients under grassland degradation. Our specific objectives were to investigate (a) the dominant response dynamics and bacterial community assemblages in degraded grasslands and (b) the dominant response strategies among various soil bacterial taxa. The results of the present study could provide a theoretical basis for further studies on microbial dynamics in degraded grasslands and facilitate grassland management activities.