# Drought and Carbon Cycling of Grassland Ecosystems under Global Change: A Review
## 2 Drought and Grassland \(C\) Cycling under Global Change
### Grazing  
Grazing has been a way of life and a common land use for people in arid and semiarid regions for centuries [114]. Grazing may have uncertain impacts on grassland \(C\) cycling [115], as it can have high intensity and relate to drought [116]. Low-intensity grazing coupled with droughts can impact aboveground productivity (AGP) and belowground productivity (BGP) [117]. Moderate grazing was less important than droughts in reducing plant cover, but heavy grazing was at least as influential [118]. Sensitivity to droughts can override grassland productivity, significantly altering \(C\) dynamics in grazing management systems [119]. The interaction between grazing and droughts can change diversity, alter dominance, and affect productivity [120]; it also restrains grass stem density and delays recovery in mesic grasslands in North America and South Africa [121]. In grazed and ungrazed North American tallgrass prairie sites, ANPP was relatively resilient under drought. Moreover, grasslands with different evolutionary histories respond similarly to grazing and drought. At frequently burned sites, droughts increase grassland sensitivity to grazing. The effects of droughts were not the primary driver of grassland productivity; instead, the magnitude of the effects is similar to those of grazing and fire [122].  
Grazing also interacts with droughts to affect BGP, and grazed grassland can be more or less sensitive to changes in precipitation than ungrazed grassland [38]. Frequently grazed grassland has a higher probability of experiencing a drought during a regrowth stage. Drought controls major trends in plant species composition and production, with grazing playing a secondary role [123]. The role of rangelands as sinks or sources of GHG, however, is determined by complex interactions between drought, vegetation, and grazing.  
Collectively, grazing and drought have uncertain effects on grassland \(C\) dynamics, contingent on the intensity of grazing and the drought as well as grassland type. In the event of extreme drought gives way to a wet year, both enhanced GPP and ER cause greater \(C\) uptake on the grazed (uptake 6 g\(\cdot\)C\(\cdot\)m\({}^{-2}\)) than on the ungrazed desert steppe (release 43 g\(\cdot\)C\(\cdot\)m\({}^{-2}\)). In an extreme year-long drought, a grazed desert steppe (release 70 g\(\cdot\)C\(\cdot\)m\({}^{-2}\)) has greater \(C\) release due to reduced GPP than ungrazed (release 48 g\(\cdot\)C\(\cdot\)m\({}^{-2}\)), but the grazed and ungrazed steppes have comparable ecosystem respiration [117]. The primary challenges in maximizing the potential of rangeland GHG mitigation are to create a complete account of GHG balances across many rangeland ecosystems and to quantify the magnitude and direction of GHG changes, due to interactions between management and environment [119]. Long-term monitoring is needed to thoroughly document mechanisms contributing to \(C\) dynamics in semi-arid rangelands in order to fully understand their role as CO\({}_{2}\) sinks, given the anticipated trajectory of future climate change [57; 124].