4.2 Different phyla taxa showed varied responses to grassland
degradation
According to the results of the present study, environmental factors
(OM, TN, and TP) significantly influenced community composition in the
degraded grassland (Figure S2) . However, the low correlation
between environmental factors and bacterial community composition means
that environmental factors can hardly explain microbial community
composition (Figure S2) ; as observed in previous studies, which
highlighted the importance of stochastic processes in microbial
community assembly (Ramette & Tiedje, 2007; Graham et al., 2016; Zhang
et al., 2016; Zhou et al., 2017).
The relationships between microbial community assembly in the
environment and ecosystem processes remain unclear (Graham et al.,
2016). In the present study, we investigated bacterial community
dynamics at phylum level in degraded grasslands. Previous studies have
reported that different soil bacterial phyla display different responses
in abundance in grasslands following shifts in nutrient conditions (Zeng
et al., 2015; Dong, Shi, et al., 2021).
In the present study, although the differences in dominant herbaceous
species and soil characteristics in the two grasslands may lead to
different belowground bacterial community composition, similar results
were observed in soil bacterial community dynamics under grassland
degradation. Proteobacteria and Bacteroidetes had more positive
responses to grassland degradation than other phyla, in the form of
richer diversity, higher abundance, and more unique and enriched species
under relatively severe grassland degradation (Figure 1 and 2;
Table 1) . The increase in the abundance of unique species and species
diversity (Table 1; Figure 2A and 2B) highlighted the
importance of Proteobacteria and Bacteroidetes, whose taxa enhanced
community function and stability under grassland degradation (Loreau &
de Mazancourt, 2013; Delgado-Baquerizo et al., 2016; Wang et al., 2017).
Moreover, Proteobacteria and Bacteroidetes exhibited greater abundance,
richness, and broader niche-widths (Figure 1, 2C, 2D, 3A and
3B ) with an increase in degree of degradation, indicating the
proportions of resources available to the two phyla may increase under
degraded conditions. Conversely, the abundance of other phyla, such as
Acidobacteria, Actinobacteria, Chloroflexi, and Planctomycetes,
decreased with an increase in degradation. In addition, although
different biomarkers were observed between the two grasslands, the
biomarkers negatively correlated with nutrients content (OM, TN, and TP)
were mainly assigned to Gammaproteobacteria, indicating the importance
of Gammaproteobacteria in the adaptation of the community to grassland
degradation. The results demonstrated that nutrient limitation in
degraded grasslands has contrasting effects on different taxa.
The responses of different taxa to their habitat environments are
associated with their ecological characteristics (Ho et al., 2017).
Proteobacteria and Bacteroidetes are considered copiotrophic taxa, and
are enriched in soil with high resource availability (Leff et al., 2015;
Wang, Zhang, Liu, et al., 2021). In comparison, oligotrophic bacterial
phyla, such as Acidobacteria and Chloroflexi, would outcompete
copiotrophs under low resource availability (Fierer et al., 2007; Ai et
al., 2015; Ho et al., 2017). However, in the present study, inconsistent
results were observed, with copiotrophic phyla (Proteobacteria,
Bacteroidetes) exhibiting greater survival, with higher abundance with a
decrease in soil nutrient contents (Figure 1) . Previous studies
have focused on stronger environmental selection associated with
artificial disturbance and larger temporal and spatial scales (Cleveland
et al., 2006; Fierer et al., 2012; Leff et al., 2015; Wang, Zhang, Liu,
et al., 2021). However, the present study focused on the effect of
grassland degradation on microbial community assembly over relatively
small spatial and temporal scales, which are associated with rather mild
shifts in resource availability. Moderate shifts in resource
availability may offer bacterial communities with opportunities to adapt
to environmental change, rather than exposing communities to strong
selection by the environment. Consistently, in the present study,
stochastic processes dominated community assembly, with weak selection
observed, based on the observed βNTI values that ranged between -2 and
+2 (Figure 4A and 4B) . Therefore, community member life
strategies may be more instructive than physiological characteristics in
predicting bacterial community assembly dynamics under degraded
grassland conditions.