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Grasslands have experienced dramatic shifts in structure and functioning driven primarily by human disturbances and global climate change (Milchunas & Lauenroth 1993; White, Murray & Rohweder 2000; Knapp et al. 2002; Wittmer et al. 2010). The Eurasian Steppe, which extends over 8000 km across northeastern China, Mongolia, Russia, Ukraine and Hungary (Coupland 1993), has been historically subjected to continuous grazing by domestic ungulates at increasingly high levels (White, Murray & Rohweder 2000). The long-term grazing has resulted in widespread declines in biodiversity and ecosystem functioning and services (White, Murray & Rohweder 2000; Bai et al. 2007). This is triggered by the direct and indirect effects of grazing and often mediated by the complex interactions between vegetation and environmental drivers (Gillson & Hoffman 2007; Golluscio et al. 2009; Shan et al. 2011). Thus, it is critical to obtain a better understanding of how grazing, abiotic factors and biotic–abiotic interactions influence key properties of ecosystem functioning and sustainability and thereby provide guideline for improving grassland management practices in the Eurasian steppe.
Abundant evidence indicates that grazing affects plant diversity, species composition and primary production (Bai et al. 2007; Sasaki et al. 2008), as well as soil properties (e.g. soil C and N pools) in many arid, semi-arid and subhumid grasslands (Frank et al. 1995; Reeder et al. 2004; Han et al. 2008). Few studies, however, have examined the mechanisms of ecosystem responses to grazing, and, in particular, the linkages between above-ground and below-ground components have received little attention (Bardgett & Wardle 2003). The grazing optimization hypothesis predicts that moderate grazing increases primary production through compensatory growth and recycling of limiting nutrients (McNaughton 1979), highlighting links and feedbacks between plant responses and nutrient cycling. Several previous studies propose that moderate grazing accelerated nutrient cycling by promoting the tissue loss of grazing-tolerant species with higher nutrient concentration, stimulating the compensatory growth and enhancing the dominance of these species, and by directly inputting urine and faeces into the system (McNaughton 1985; Frank & Evans 1997). Other studies, however, have shown that grazing decelerated nutrient cycling by inhibiting the growth of palatable and nutrient-rich species with high litter quality and promoting the dominance of those nutrient-poor or chemically defended species with low litter quality that slow rates of nutrient cycling (Ritchie, Tilman & Knops 1998). These seemingly contradictory hypotheses suggest that the effects of grazing on nutrient cycling and ecosystem functioning are contingent on a host of processes, including vegetation type (dry vs. mesic grasslands), evolutionary history of grazing (grazing tolerance of plants), species composition (e.g. plant tissue nutrient concentration, palatability), grazing intensity and N inputs and outputs of the system (Milchunas, Sala & Lauenroth 1988; Ritchie, Tilman & Knops 1998; Singer & Schoenecker 2003).
Ecological stoichiometry provides a powerful framework for studying how grazing affects the balance of essential nutrients (e.g. C, N and P) at different trophic levels and over a wide range of spatial and temporal scales (Sterner & Elser 2002). Two mechanisms of how grazing affects the C, N and P contents and stoichiometry at the community level have been proposed (Bardgett & Wardle 2003). First, grazing may change the contents and stoichiometry of the plant community through a cascade of plant–soil feedbacks (McNaughton 1985; Frank 2008). Grazing often increases C-rich root exudates that stimulate microbial activity and turnover, ultimately resulting in an increase in soil nutrients available to plants (Bardgett, Wardle & Yeates 1998; Hamilton & Frank 2001). However, these stimulatory feedbacks cannot be sustained at high grazing intensities and may be absent or weak in resource-poor conditions. Second, grazing may alter the species composition and thus stoichiometry of the community because species differ in their nutrient contents (Ritchie, Tilman & Knops 1998; Bardgett & Wardle 2003).
Spatial scale is an important dimension for understanding mechanisms underpinning the observed responses in ecosystem functioning (e.g. primary production, C, N and P pools and stoichiometry). This is because the direction and magnitude of these effects are system and/or scale dependent (Levin 1993; Milchunas & Lauenroth 1993), and they often interact with environmental conditions, such as water and N availability (Maschinski & Whitham 1989; Wise & Abrahamson 2005). Few studies have examined the effects of grazing on ecosystem functioning and C : N : P stoichiometry across broad geographic regions. Understanding these complex relationships requires large-scale reference sites that have not been grazed. The border between China and Mongolia, with a demilitarized fenced area preventing grazing, provides an ideal transect to address this deficiency because the buffer zone has not been grazed since the 1950s. The arid and semi-arid grasslands on the Mongolia plateau, representative of the Eurasian steppe region, are extremely water-limited, with a similar evolutionary history of grazing (Coupland 1993; Bai et al. 2008). However, it remains unclear to what extent the water availability affects functioning and stoichiometric responses of grasslands to long-term grazing across broad regions.
To test these hypotheses and predictions, we examined the effects of grazing on ecosystem functioning and C : N : P stoichiometry along the 700 km China–Mongolia transect (CMT) using consistent methods. The CMT, which covers a wide range of biotic and abiotic conditions, enables us to observe the total effects of multiple mechanisms that probably operate simultaneously but vary in their relative strengths across regions. Specifically, we address the following questions: (i) How has grazing affected ecosystem functioning (i.e. species richness, above- and below-ground biomass and litter biomass) and C : N : P stoichiometry of grasslands along the regional precipitation gradient during the last 50 years? (ii) How do the responses of plant and soil C, N and P pools and stoichiometry to grazing differ among community types? (iii) What is the relative importance of plant functional group (PFG) composition and species plasticity in influencing ecosystem functioning and stoichiometry?
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Fig. S1. Effects of grazing on C, N, and P contents (%) in above- and below-ground biomass, litter, and soil across different community types of the China–Mongolia transect (Error bars represent SE).
Fig. S2. Effects of grazing on foliar C, N, and P contents of dominant and subdominant species (error bars denote SE).
Table S1. Abiotic and biotic characteristics of the 18 paired study sites along the China–Mongolia transect.
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