General function and specific function with community-level
diversity
Our results unequivocally showed that microbial community diversity
enhanced ecosystem broad functioning, and had a proportional effect with
no functioning redundancy among land use types (Fig. 3a). These
observations are consistent with previous reports of positive effect of
microbial richness on ecosystem functioning (Bell et al. 2005;
Delgado-Baquerizo et al. 2016b). However, these previous studies
did not explicitly check for the relationship of the specialized
microbes’ richness with their function.
In this present study, the
empirical evidence demonstrated that any loss in microbial diversity
could reduce multifunctionality, but the specialized microbes did not
follow the significant correlation with its functions in afforested
lands (Fig. 3c, e).
The different patterns between the broad and specialized BEF
relationship could be explained by the below reasons. Generally,
microbial respiration is an activity of active microbes’ consumption
soil carbon and important energy acquisition pathway, while the existing
of specialized microbes may rely on environmental context
(Delgado-Baquerizo et al. 2016a; Trivedi et al. 2019a).
For example, previous studies reported that the richness ofNitrososphaera ( affiliate to AOA) were drastically
increased and the obligate N fixers were detrained from species pools
under nitrogen fertilization scenery, with no significant changes ofNitrosospira (affiliate to AOB) (Shen et al. 2012; Gaoet al. 2018). Notably, we found that the effect of microbial
community on the specialized function was stronger compared to the
abiotic factors in the croplands (Table. S1), in this case that there
were high energy and nitrogen deficit for soil specialized microbes
under low nutrient status (Table S1; (Delgado-Baquerizo et al.2016a; Feng et al. 2018).
Another reason could be low
resolution of the broad microbes.
Although the active microbes can
be detected by the PLFAs, the difference in the phospholipid markers was
not enough to distinguish the probability of community redundancy, thus
a small change in the richness of phospholipid markers could lead to
changes in the broad functioning (Zhang et al. 2016; Veumet al. 2019). Additionally, we found different patterns between
the specialized microbes (AOA and AOB), namely, the AOA communities did
not exhibit direct and significant relationship with the NR (Fig. 6b,
c). As noted previously, low rates of ammonium supply and alkaline soil
in our study site could be a benefit for the AOB keystone species
(Nitrosospira , Table S4) (Ke et al. 2013; Gao et
al. 2018).