4.1 The properties of soil microbial community degradation
In this study, microbial community degradation was accompanied by significant variation in community composition, α-diversity and functions. The α-diversity rose and then decreased again, which indicated the potential error to describe microbial community degradation by α-diversity as a proxy (Fig. S2). Degradation is triggered by stresses (Li et al. , 2013), which alters soil properties in a spatially and temporally heterogeneous manner (Pedersen et al. , 2015, Jianet al. , 2018, Li et al. , 2020), creating new niches that could harbor new species (Seaton et al. , 2020). However, with increasing degradation, soil properties are more likely to be altered to an extent that the number of niches decreases, filtering species (Orrock, 2020). Thus, the relationship between α-diversity and degradation varies from the extent (or stage) of degradation.
We found that during degradation, microbial functions focused more on stress resistance and basic metabolite, and less on interaction and reproduction (Fig.1). This indicated microbial community was under pressure such that its members reduced energy utilization efficiency to overcome the stressor(s) (Malik et al. , 2018). When facing acidic stresses, microbes expend extra energy to synthesis stress tolerance protein and, thus, leaving less energy for reproduction (Malik et al. , 2018). Further, our results supported that the microbial community lost metabolite cooperation during degradation as shown by the decrease in the relative abundance of gene modules related to microbial interaction and increase in the relative abundance of gene modules related to basic metabolism, which coincided with the decrease in microbial community interactivity (Fig.1, Fig.2B and Table S2). This indicate that under UD, members in the network were able to conserve energy by scaling back the synthesis enzymes and proteins for entire pathways, achieve high energy acquisition and reproduction rate through the exchange of intermediate metabolites (Damore & Gore, 2012, Maliket al. , 2018). The broken metabolite cooperation under stress conditions forced members to express a great number of basic metabolite related gene to ensure survival (Damore & Gore, 2012, Malik et al. , 2018). We found the inhibited metabolite cooperation occurred more frequently between fungi and bacteria than between fungi and fungi or bacteria and bacteria, since the positive interaction ratio of the former consistently decreased over the degradation gradient (Fig.3A). In nature, fungi play an important role in litter degradation and contribute the main driving force for organic carbon release to the soil (Taylor & Osborn, 1996), while bacteria play a larger role in various ecosystem functions such as ammonia oxidization and nitrogen fixation. (Galloway et al. , 2004). The decreased cooperation between fungi and bacteria is a potential indicator of decoupling between their respective functions (Taylor & Osborn, 1996, Galloway et al. , 2004, Kuypers et al. , 2018). This was further supported by the loss of inter-module interactions (Fig. 2A and Fig. 2B), since members within the same module are more likely to be linked to a single function, decreasing inter-module interaction indicates decoupling of different functions (Deng & Zhou, 2015). As discussed above, our result support that the interactivity, rather than simple diversity, is better able to indicate microbial community degradation. Due to the extremely high richness and functional redundancy of the microbial community, it is more meaningful to understand who’s working or interacting together rather than who is simple present (Curtis, 2006).