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.