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.