Introduction
Soil microorganisms participate in almost all biogeochemical cycles and are extremely important for maintaining soil ecosystem functions (Bardgett & van der Putten 2014; Wagg et al. 2019; Zhou et al. 2020), whilst a large body of literature has documented that soil ecosystem functions can be threatened by land use change possibly due to loss of microbial biodiversity under land disturbance (Cardinaleet al. 2012; Allan et al. 2015). For instance, previous studies have found that high microbial diversity is of stability via recruitment from the indigenous taxa pools, and hence land use change may lead to alpha diversity in decoupling of plant and microbial community (Allan et al. 2015; Zhou et al. 2020). While land use change, as a key component of global change, has occurred worldwide, particularly, recent land use change has induced dramatic alterations in vegetation type and microbial attributes (Cardinaleet al. 2012; Cheng et al. 2013; Allan et al. 2015), and thus concurrently can affect belowground biodiversity and ecosystem functioning. However, the relationship between soil microbial biodiversity and ecosystem functions in response to land use change remains largely uncertain due to soil microorganisms’ huge diversity with the taxa ranging from hundreds to thousands per gram of soil (Torsvik & Øvreås 2002; Bardgett & van der Putten 2014).
Numerous studies conducted with plants and microbes support a consensus view that the relationship between microbial biodiversity and ecosystem functioning (i.e. BEF relationship) is the fundamental in addressing global environment changes (Tilman et al. 2014; Isbell et al. 2015). Some studies have generated that ecosystem functions decrease with the loss of biodiversity in the experiments and observation (Tilman et al. 2014; Meyer et al. 2018), as well as in the meta-analyses (Cardinale et al. 2012; Delgado-Baquerizo et al. 2016b; Zhou et al. 2020). Generally, the linear BEF relationship suggests that each species has a proportional effect on ecosystem function with no functional redundancy (Delgado-Baquerizo et al. 2016a; Trivedi et al. 2019a). Whereas, the logarithmic BEF relationship indicates that a small loss of biodiversity has a minimal or none impact on ecosystem due to the functional redundant (Louca et al. 2016; Trivedi et al.2019a).
Soil ecosystem functions such as the board (widely dispreads living microbe, e.g. decomposition) and specialized (i.e. ammoxidation) levels are critical for ecosystem energy gaining and nitrogen supplying processes (Bender et al. 2016; Manning et al. 2018; Zhouet al. 2020). And determining the BEF relationship at the broad and specialized level is of pivotal significance to estimate the impact of diversity losses induced by land use change on ecosystem functioning such as soil carbon sequestration or decomposition (Allan et al.2015; Delgado-Baquerizo et al. 2016a; van der Plas 2019). It has been argued that these two ecosystem functions are known as relatively independent microorganisms’ communities (Srivastava & Vellend 2005; Delgado-Baquerizo et al. 2016a). For microbial keystone taxa usually drive community richness and function irrespective of their abundance (Banerjee et al. 2018), consequently, changes in keystone taxa can affect soil ecosystem function and reconstruct the BEF relationship. A big challenge is how to explicitly identify the relationship of the board and specialized functions with microorganisms in response to land use change. Although most of these interactions of microbes living network cannot directly observe, the network-based analytical approach is a useful tool to infer the microbial interaction and keystone species of the complex network (Deng et al. 2012; Weiss et al. 2016). Thus, the construction of a co-occurrence network enables our disentangling the BEF relationship under land use change.
In this study, we identified the BEF relationship under land use change in the Danjiangkou Reservoir area in central China. After the project of Middle Route of China’s South-to-North Water transfer Project was constructed, large areas of cropland have been converted to shrubland and woodland since 1980s (Cheng et al. 2013). Our previous studies have demonstrated afforested lands enhanced soil quality, and microbial activities (Cheng et al. 2013; Feng et al.2018). Therefore, the variations in soil microbial communities and diversity are expected across over 30 years of land use change in our study site, which is useful to evaluate the relationship between microbial diversity for both broad (i.e. microbial basal respiration) and specialized function (i.e nitrification rate). Here, we hypothesize that: (i) the pattern of BEF relationship would vary with across land use change in the status that land use change would substantially alter soil microbial communities; (ii) considering for environmental changes across different land use types, the role of maintaining nitrification rate shifts between AOA and AOB possibly due to niche differentiation.