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).