General function and specific function associated with keystone species diversity
The keystone species have been proven to exert a predominant ecological function in microbial functions (Banerjee et al. 2016; Trivediet al. 2017b; Banerjee et al. 2018). The keystone species showed significant negative correlations with the broad functions (Fig. 4; Fig 6b, c), which could be expected that the broad functions were maintained by more free-living populations (Hooper et al. 2005; Fan et al. 2018). And the higher proportion of negative relationship of the PLFAs network indicated the negative feedback primarily dominated microbe communities with less dynamic structure and greater ecological stability (Fig. S3). In addition, the maintains of community richness was a foundation of the broad function irrespective of land use types (Fig. 4). To some extent, our studies demonstrated the broad functions were determined by community richness and irrespective of environmental changes (Isbell et al. 2015; Delgado-Baquerizoet al. 2016b).
Inconsistent with the results observed for broad function, we found a lack of relationship of between keystone species and specialized functions except the AOA community in the cropland (Fig. 3d). Meanwhile, the bacterial network hubs, the keystone OTUs composition belonged to Nitrosospira was slightly related to the PNR (Fig. 4c). The present results demonstrated that losses of some specific groups of AOB could collapse the metabolic routes at specific environments (Fig. 4c) (Liu et al. 2019). This finding was inconsistent other observations with no context-dependence (Delgado-Baquerizo et al. 2016a; Trivedi et al. 2019a), which results showed little functional redundancy in the relationship between the functional genes diversity and specialized ecosystem functions from independent sites. By comparing other studies (Trivediet al. 2017a; Banerjee et al. 2019) with natural assembled communities in our study, the keystone species composition under the dilution-to-extinction approach may not fundamentally change, which has a disproportionate effect on ecosystem functions (Berry & Widder 2014; Trivedi et al. 2019a). According to the traditional paradigms (niche complementarity and sampling effects) (Tilman et al. 2001), some studies documented that the increased functional rates observed were ascribed to the presence of particular species, rather than to an increase in species diversity (Strong et al. 2015; Daam et al. 2019). Consequently, in low substrate status, the lack of biodiversity-functioning relationship of the specialized function could be related to the community composition of keystone species (Strong et al. 2015; Daam et al. 2019).
In conclusion, the BEF relationship showed different patterns in the broad function and specialized function. The broad function was maintained primarily by the community-level diversity, although the keystone species played a large role in the broad function, but the weak relationship indicated diversity effect (niche compensation) could dominate this process and community richness recruitment from other free-living species. In contrast, the relationship between the specialized function and keystone species composition, nor theamoA richness was observed in the afforested soils. These results suggest that the keystone species can maintain ecosystem function through identity effect. More importantly, we found that land use change did not alter the robust biodiversity-functioning relationship, but revealed more sensitivity of diversity loss in the afforested soils. However, land use change directly regulated the NR by environmental factors (particularly, soil temperature), and affected the PNR by regulating keystone species. Collectively, unlikely the classical positive but decelerating biodiversity-functioning relationship (Tilmanet al. 2014; Delgado-Baquerizo et al. 2016a), our finding provided direct evidence the positive relationship only existed in the broad functioning, but the specialized functioning depended on land use scenario. These results further reveal the different underlying mechanisms in maintaining the relationship of the broad and specialized function with the microbial diversity, highlighting the important role of keystone species in corresponding specialized soil functions (e.g. nitrate availability) to harness their potential for predicting modelling of biochemistry cycle under global land use change.
AcknowledgementsThis work was supported by the Strategic Priority Research Program B of the Chinese Academy of Sciences (XDB15010200), the National Natural Science Foundation of China (31770563) and the China Postdoctoral Science Foundation (2019M663587). We declare no conflict of interest.