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.