Introduction
Earth’s ecosystems have been severely affected by anthropogenically driven environmental change (Barnosky et al. 2012), causing dramatic biodiversity loss (Díaz et al. 2019) and, consequently, threatening their capacity to provide ecosystem functioning and services (Cardinale et al. 2012; Brose & Hillebrand 2016; Díaz et al. 2019). Grasslands are a globally important biome that comprises approximately 22% of the Earth’s ice-free land (White et al.2000), and provides an important contribution to global food supply and security (O’Mara 2012). Grasslands are the most endangered terrestrial ecosystem type due to overexploitation by rapidly increasing human population and consumption (Millenium Ecosystem Assessment 2005), and are more vulnerable to environmental change than some other ecosystems (Hoekstra et al. 2005). Therefore, understanding how grasslands respond to environmental change and land-use is extremely important for global sustainability. However, the combined effects of climate change and land-use on biodiversity and ecosystem functioning are still poorly understood (Siebert et al. 2019).
Biodiversity has been increasingly recognized as a major determinant of ecosystem dynamics and functioning (Tilman et al. 2014), partly because species vary in their contributions to different functions (Isbell et al. 2011; Meyer et al. 2018). Empirical evidence comes from both experimental and observational studies (Duffyet al. 2017), and indicates that complementarity effects mainly explain the positive effects of biodiversity on ecosystem functions (Loreau & Hector 2001; Reich et al. 2012). For example, plant communities with higher diversity are able to capture more resources due to dissimilar resource use strategies, convert them into new biomass, and store more carbon (Hooper et al. 2005; Cardinale et al. 2011; Carol et al. 2018). Indeed, recent studies indicate that effects of environmental change and human disturbances are mediated by changes in biodiversity (Isbell et al. 2013; Allan et al. 2015; Dainese et al. 2019; Beaumelle et al. 2020). However, the relative importance of different, co-occurring environmental change drivers in driving biodiversity-mediated changes in ecosystem functioning and their potential context-dependencies have rarely been studied (Eisenhauer et al. 2019; Giling et al.2019). Biodiversity effects may depend on the environmental context and stress (Guerrero-Ramírez et al. 2017; Ratcliffe et al.2017), driving the balance between competitive and facilitative interactions (Wright et al. 2017).
Aridity is the most important abiotic factor influencing many grassland ecosystems, because most grasslands are water limited (Knapp & Smith 2001; Merbold et al. 2009; Harpole et al. 2011). The projected increases in aridity will diminish the ability of global grasslands to provide life-supporting ecosystem services (Dai 2013; Sherwood & Fu 2014; Trenberth et al. 2014; Berdugo et al.2020). Aridity has been reported to reduce plant species richness and productivity, and alter the structure of above- and belowground communities from local to global scales (Tilman & EI Haddi 1992; Zhao & Running 2010; Maestre et al. 2015; Berdugo et al.2020). Because ecosystem functions, such as nitrogen cycling, carbon sequestration, and litter decomposition, are regulated by these biotic attributes, ecosystem functioning is likely to decline with increasing aridity (Maestre et al. 2012; Delgado-Baquerizo et al.2013; Berdugo et al. 2017; Durán et al. 2018; Beaumelleet al. 2020). Furthermore, land-use strategies have been found to modulate the response of grasslands to climate change (Jonas et al. 2015; Karlowsky et al. 2018; Stampfli et al. 2018). Yet, how land-use and aridity interactively affect grassland biodiversity and ecosystem functioning has been rarely explored, particularly so at large spatial scales.
Livestock grazing and haying are prevalent land-use types in global grasslands, and occur across virtually all biomes from arid to wet climates (Asner et al. 2004; Steinfeld et al. 2006; Erbet al. 2017). The growing human population and mounting food demands require an increase of livestock production (Bouwman et al. 2005; Tilman et al. 2011), which will likely lead to higher grazing and haying intensity in natural grasslands. Grazing, especially by native ungulates, and haying have been reported to facilitate plant diversity and productivity and increase soil carbon (C) and nitrogen (N) stocks (Sirotnak & Huntly 2000; Socher et al. 2012; Conget al. 2014; Wang et al. 2019). However, overgrazing and intensive haying can reduce plant diversity and biomass as well as soil functions, and exacerbate grassland degradation (Eldridge et al.2016; Eldridge et al. 2017). The stocking rates and grazing intensities that plants experience under managed grazing by livestock can be extremely high relative to the levels of grazing by native ungulates that plants experienced throughout their evolutionary histories (Milchunas & Lauenroth 1993). For example, herbivore biomass per unit primary productivity can be an order of magnitude greater in managed livestock grazing systems than in natural grazing systems (Oesterheld et al. 1992). Further, the influence of grazing on grasslands will be altered with environmental change. For instance, in arid lands, grazing is a pervasive driver of loss of soil functions (Bridges & Oldeman 1999), resulting in reductions in soil organic C and aboveground biomass (Gallardo & Schlesinger 1992; Asner et al.2004). In semiarid regions, grazing could lead to woody encroachment and thus to an increase in both above- and belowground carbon stocks (Eldridge et al. 2011; Anadón et al. 2014). A global meta-analysis revealed large differences in the response of C3- and C4-dominated grasslands to grazing under different rainfall regimes (McSherry & Ritchie 2013). Therefore, grazing and haying may have context-dependent effects, which is why the ecosystem consequences of land-use on grasslands need to be examined in a wide range of environments.
Here we examine how plant species richness, plant biomass, and soil C concentrations respond to a spatial gradient of aridity (ranging from 0.25 to 0.89, covering major global ecosystem thresholds; Berdugoet al . 2020) and land-use (grazing and haying) at 716 sites along an extensive environmental gradient of 2,200 km across China (Fig. S1). This region is one of the major sources of animal products in China and acts as a natural green barrier towards the vast cropland regions, thus having crucial ecological and economic importance (Kang et al.2007). We hypothesized that (1) aridity and land use will decrease plant diversity, (2) ecosystem functioning would increase with increasing plant species richness, (3) effects of aridity, grazing, and haying on ecosystem functioning would be mediated via variation in plant species richness.