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).