4.3 Taxa strategies in response to grassland degradation
Considering the microbial strategies adopted in response to grassland
degradation, the life strategies of bacterial phyla observed in the
present study were classified into r -and K -selection
strategies, based on the life-history evolution theory by Pianka
(Pianka, 1970). r -selection strategy is associated with high
productivity, and is adopted to improve survival chances under variable
and unpredictable environmental conditions, in contrast toK -strategy that is associated with high efficiency, and is
adopted to maintain survival ability in response environmental stress
(Pianka, 1970; Reznick et al., 2002; Ye et al., 2018). In addition,r -extreme represents a quantitative extreme with no density
effects and no competition, whereas K -extreme represents a status
with active competition and is the optimal strategy for allocating
limited resources to ensure resource efficiency, for example, by
producing a few extremely fit offspring (Pianka, 1970; Ye et al., 2018).
No organism adheres to either r -extreme or K -extreme
completely, and communities must achieve a balance between the two
extremes in response to environmental disturbance (Pianka, 1970; de
Vries & Shade, 2013).
In the present study, Proteobacteria and Bacteroidetes had greater
species diversity, broader niche-widths, and random phylogenetic
clustering in degraded grasslands (Table 1; Figure 3A, 3B, 4C
and 4D) , showing high adaptability to the resource limitation, which
can be considered adoption of r -strategy. The finding is
consistent with the results of a previous study, in which Proteobacteria
could survive in diverse ecological niches due to their variable
morphology and physiology, which facilitate an r -strategy (Shin
et al., 2015). Conversely, other phyla (mainly Acidobacteria,
Actinobacteria, Planctomycetes, and Gemmatimonadetes) had lower
diversity, abundance, and niche-overlap (Table 1 and 2; Figure 1
and 3C and 3D) so that bacterial abundance was reduced following
functional redundance to enhance resource-use efficiency, which can be
considered K -selection. Similar results have been reported in a
study on the relationship between phyla-level responses and life
strategies under drying-rewetting disturbance, in which Proteobacteria
employed opportunistic strategies (r -selection) whereas
Acidobacteria adopted tolerant strategies (K -selection) (Evans et
al., 2014).
Although the r /K strategy theory is oversimplified, it provides
researchers with a fundamental ecological perspective for understanding
mechanisms of assembly of microbial communities in response to
environmental change (de Vries &
Shade, 2013; Li et al., 2021). The increases in the relative abundances
of Proteobacteria and Bacteroidetes with grassland degradation indicate
that the bacterial community adopted r -selection strategies.
Generally, r -selection strategies are more adaptable to variable
environmental conditions than K -selection strategies (Ye et al.,
2018); furthermore, high variable productivity observed underr -selection potentially offers more opportunities for community
evolution (Fierer et al., 2012) in conditions such as degraded
grasslands with nutrient loss.
Selection strategies can also be reflected in community functional
structure, which is correlated with community response to environmental
change (Schimel & Bennett, 2004; Fierer et al., 2012; de Vries &
Shade, 2013). In the present study, bacterial community functional
diversity and functional information were enriched with nutrient loss(Table S4 and S5) . Moreover, phylogenetic dispersion was
significantly and positively correlated with the number of KO genes
carried (Figure 5) , highlighting the potential importance of
stochastic assembly processes in the regulation of community functional
structure. The community assembly and niche analysis results in the
present study jointly indicate bacterial communities could directionally
regulate their functional structure based on diversity and composition
under grassland degradation, which could facilitate their adaptation to
changes in resource availability.
We do not exclude the influence of species-competition and/or
environmental filters on the community composition, because microbial
response are likely affected by multiple mechanisms simultaneously (Ho
et al., 2017). Environmental selection was potentially weak due to the
rather mild changes in resources in the course of grassland degradation
over the relatively small spatial and temporal scales. Nevertheless,
environmental factors can indirectly influence community response
strategies in the form of community assembly (Fierer et al., 2007; Li et
al., 2021). The balance between r - and K - strategies
mediated by community members of different phyla facilitates adaptation
to resource loss under grassland degradation (Figure 6) .
However, following long degradation periods, shifts in environmental
nutrient conditions could induce shifts from r - toK -selection strategies, considering the accumulation of
environmental selection and the strengths of oligotrophic bacteria
following environmental filtering (Pianka, 1970). Simultaneously, the
relative importance of stochastic process may decrease, with the
deterministic processes playing more dominant roles in structuring
communities (Ferrenberg et al., 2013; Zhang et al., 2016).
5. Conclusion
Stochastic process with weak selection dominated bacterial community
assembly process during grassland degradation, with mild and continuous
environmental changes on a small spatial scale. Different bacterial
phyla exhibited different response strategies in the form of diversity,
abundance, niche-width, and phylogenetic turnover under grassland
degradation (Figure 6) . Proteobacteria and Bacteroidetes, the
most abundant phyla, exhibited positive responses, adoptingr -strategies, which facilitated the enhancement of biodiversity
and in turn community productivity. In contrast, the relative abundances
of other dominant phyla were restricted following grassland degradation,
with K -strategies being adopted to facilitate efficient resource
utilization. The shift from K - to r - strategies by
communities under grassland degradation brought increased functional
diversity, which could facilitate adaptation by communities to resource
limitation. A key limitation of our study is that not all phyla can be
classified as r or K strategists (Li Hui, 2021);
nevertheless, the system can still be employed to partly explain the
response mechanisms of microbial communities to environmental changes.
In addition, although we determined the influence of community response
strategies on functional structure, the specific functional structure
dynamics involved in community adaptation to grassland degradation
require further research. In summary, our study clarified the assembly
processes of soil bacterial communities in response to grassland
degradation, and the results offer insights into the ecological dynamics
involved in the responses of various soil bacterial communities and taxa
to grassland degradation. The findings of the present study could
facilitate not only the prediction of soil microbial community structure
in grassland ecosystem under degradation but also their sustainable
management.