# Drought and Carbon Cycling of Grassland Ecosystems under Global Change: A Review
## 2 Drought and Grassland \(C\) Cycling under Global Change
### Drought and Grassland C Cycling  
Water is a limiting factor in grasslands, many of which experience periodic droughts [10; 24]. On a global scale, the frequency, duration, and intensity of droughts have increased strongly in recent decades [25], especially in arid and semi-arid regions [7]. Droughts are the main source of inter-annual variation in terrestrial \(C\) sequestration, as they cause large reductions in GPP as well as in the net ecosystem exchange (NEE) of terrestrial ecosystems [26; 27]. Droughts also have negative effects on soil biodiversity, the content of SOM, and water retention [28]; they may have implications on ecosystem functions that last longer than the drought itself [29; 30]. Most notably, the ecosystem \(C\) sequestration accumulated over a number of years could be undone by a single severe drought [26; 31]. When severe droughts occur, soil microbes struggling for 'food' resources eat into the 'old' soil \(C\) accumulated over a number of years in grasslands [32]. Additionally, under future climate scenarios, productivity is predicted to increase in North American grasslands despite rising aridity [33]. Therefore, droughts are key determinants of \(C\) cycling in grassland ecosystems, as they alter water provision and use [34; 35]. \(C\) cycling has different responses to various drought intensities, as is supported by many studies [36; 37; 38]. Depending on their intensity, frequency, duration, and timing, not all droughts have an equal impact on grassland \(C\) cycling [39]. Drought intensity partly determines the fate of \(C\) allocation for photosyntheate in grassland plants [40]. However, extreme drought limited \(C\) translocation from aboveground to belowground storage, while a moderate drought provoked allocation, assigning \(C\) translocation to the area of the plant where it was most urgently needed at high water stress [41]. Figure 1: The schematic diagram describes the complex interaction between drought and global change drivers on grassland \(C\) cycling. In the paper, global change drivers include elevated CO\({}_{2}\) concentrations, global warming, \(N\) deposition, grazing, and land-use changes. To some degree, the chronic and gradient factors can counteract the negative effects of drought on grassland \(C\) cycling via their positive effects driven by climate change, such as elevated CO\({}_{2}\) concentrations, global warming, and \(N\) deposition; on the other hand, the periodic and pulsing factors can amplify the negative effects of drought on grassland \(C\) cycling via human activities such as grazing and land-use change. In grasslands, the ecosystem has the ability to buffer impacts from drought and large amounts of soil \(C\) in its surface layers. As climate change and human activities continue, droughts will have a stronger and more complicated effect on \(C\) cycling in grassland ecosystems in the future. In another study, soil CO\({}_{2}\) flux decreased by 8% under reduced rainfall amounts, by 13% under altered rainfall timing, and by 20% when both were combined [42]. Thus, it was suggested that drought timing was a factor more critical than intensity in affecting \(C\) dynamics in semi-arid regions [43; 44; 45]. Similarly, the developmental stage of a plant could be determined by a response to drought [46]. Seedlings are known to be highly susceptible to drought stress [47]. A shift in spring drought impacts the structure and function of grasslands more than a drought in summer/fall for the North American Great Plains [48]. Furthermore, drought has mixed effects on grassland ecosystem processes. In the short term, drought adversely affects root biomass, litter decomposition rates, and short-term CO\({}_{2}\) fluxes, but increases soil nutrient retention, soil fertility, and longer term \(C\) fixation rates [49]. In addition, droughs may have different impacts on aboveground and belowground productivity or communities [50].