Factors indirectly and directly regulate ecosystem functions
We also explored the direct and indirect effects of environmental
factors on ecosystem functions under land use change (Fig. 6), which are
pivotal for better understanding the underlying mechanisms of
diversity-ecosystem functioning relationship under varying environments
(Brose & Hillebrand 2016; Peters et al. 2019; Wang et al.2019). We found that the substrate supplying factors (e.g. TC, TN, and
C/N) were directly associated with the mSR (Fig. 6a), highlighting the
evidence that substrate quality dominated ecosystem functions (Nieet al. 2013; Zhang et al.2016). It has been well
established that increased soil C and N supply could stimulate microbial
activities as they were sources of nutrients and energy to
microorganisms (Kaiser et al. 2016; Yang et al. 2018).
Although environmental factors, especially soil pH has been considered
as best predictor of microbial community composition (Fierer 2017; Yanget al. 2018) and ecosystem functioning (Delgado-Baquerizoet al. 2017; Zhou et al. 2020), here it was evidenced that
the indirect environmental factors predominately controlled over
ecosystem broad function by shifts in microbial community richness
(Figure 6). The possible explanation for the indirect effect was that
environmental factors span a narrow range, and did not rapidly respond
to land use change (Fierer 2017). Thus, the positive relationship
between the substrate supplying and microbial broad functions suggested
that the microbial respiration was probably both energy- and nutrient-
limited across land use types in our study site (Trivedi et al.2016; Zhang et al. 2016).
By comparing ammonium availability in yellow-brown soil of our study
site (0.8 mg/Kg, low level) to black soil (1.89 mg/Kg) of other studies
e.g. (Liu et al. 2018), we inferred the PNR was maintained by
substrate supplying factor irrespective of alterations in the microbial
communities. However, in our study, the NR was directly related with
environmental factors (especially, soil temperature), and the PNR was
directly regulated by the AOA and AOB keystone species community (Fig.
6b, c). This result accorded with a previous study that found soil
temperature was the important driver of global soil nitrification rate
(Li et al. 2020). Some studies have observed niche
differentiation between the AOA and AOB communities across different
land use types (Dai et al. 2018; Hink et al. 2018; Liuet al. 2018). The inconsistency between our results and previous
hypothesis indicated that land use changes in alkaline soil did not
significantly affect the AOA and AOB community diversity (Fig. 2). This
founding could be explained by a previous review with neutral pH and low
nitrogen availability (Shen et al. 2012).