Relationship between plant richness and ecosystem functioning
In accordance with our hypothesis, aboveground biomass and SOC were positively associated with species richness in this study. This is consistent with previous biodiversity and ecosystem function (BEF) work (e.g. Ma et al. 2010; Maestre et al. 2012; Pasari et al. 2013; Wagg et al. 2014; Jing et al. 2015). Though there is still debate about the mechanisms by which biodiversity impacts ecosystem function (Eisenhauer et al. 2019), many studies have demonstrated that biodiversity is a major determinant of ecosystem functioning (Isbell et al. 2011; Tilman et al. 2014; Isbell et al. 2017; Seabloom et al. 2017). Moreover, there have been several studies showing that positive biodiversity effects on ecosystem functioning are particularly strong when multiple ecosystem functions are considered (Isbell et al. 2011; Maestre et al. 2012; Pasari et al. 2013; Jing et al. 2015; Van Der Plas et al. 2016; Fanin et al. 2018; Hautier et al.2018; Meyer et al. 2018). Our study, across an extensive aridity gradient, complements the growing BEF literature by demonstrating strong plant diversity effects on both plant biomass production and SOC (Table 1, Figs. 3 and 4). Although the present study was not designed to identify the underlying mechanisms, based on previous work we speculate that positive effects of species richness on ecosystem functions may be mediated by complementarity in the resource use and increased plant productivity, and cascading effects on multiple organisms and ecosystem processes (Pringle et al. 2007; Maestre et al. 2012; Tilman et al. 2014). Notably, in this study, plant species richness had greater effect on SOC than on aboveground biomass, underlining the significant role of plant diversity for soil carbon storage in grassland (Lange et al. 2015).
In addition, the positive relationships between plant species richness and aboveground biomass as well as SOC were stronger at more arid sites (Fig. 3). This suggests that plant diversity has stronger beneficial effects on ecosystem functioning in dry regions, supporting the hypothesis that biodiversity may be particularly important to maintain ecosystem functions under stressful environmental conditions (Callawayet al. 2002; Ratcliffe et al. 2017; Baert et al.2018). In the more humid regions, the weaker effects of species richness on ecosystem functioning may result from reduced complementarity effects (Wright et al. 2017) and increased functional redundancy at high diversity (Craven et al. 2016). These results have significant implications for land management in a changing climate, as they indicate that the preservation of plant diversity is particularly important to maintain ecosystem functioning and services in arid environments.