# Drought and Carbon Cycling of Grassland Ecosystems under Global Change: A Review
## 3 Syntheses and Perspectives  
### Combined Effects of Multiple Factors  
Ecosystems exist in a multi-factor world. Global change drivers can mediate the rate and efficiencies of both photosynthesis and water use to affect ecosystem productivity and other processes. Realistic combinations of global change drivers show small diversity effects, but a remarkable effect on dominant species [159]. In addition, global change drivers can alter available resources in an ecosystem by directly affecting the biota and driving ecosystem responses, causing chronic changes in water balance and modifying the biogeochemical cycle of nutrients [160]. Drivers can also change ecosystem nutrient dynamics indirectly by affecting plant litter quality [161]. The natural ecosystem response to global change drivers may be constrained by different perturbations [162]. Collectively, current disturbance regimes and global changes impose a suite of impacts on available resources in an ecosystem [163]. Effects from global change drivers and droughts may interact within ecological diversity and in the composition of natural communities [164]. \(C\) cycling may mitigate or exacerbate climate change, depending upon the relative responses of grassland \(C\) sequestration and emissions to global change factors and droughts [165]. Climate, fire, and grazing are three important drivers affecting the composition, structure, and function of grasslands. There are also many interactions between these drivers, which affect ecological patterns and processes in grasslands differently than if they were single drivers [166]. Grassland composition and diversity are primarily governed by long-term regional climate. To some extent, grassland composition and diversity can alleviate the impacts of a drought on an ecosystem [121]. In a warmer world, elevated CO\({}_{2}\) concentrations will increase both soil water content and productivity in semi-arid grasslands [167]. Adversely, elevated CO\({}_{2}\) alone, or in combination with warming, alters ER to a greater extent than GPP, resulting in net \(C\) loss by the stimulated decomposition of SOM. For instance, an elevated CO\({}_{2}\) concentration causes a greater increase in \(C\) cycling than in \(C\) storage in grasslands [168], which can alleviate or offset the impacts of a drought on \(C\) cycling. Also, anthropogenic pressures such as grazing and land-use change could be key drivers of biodiversity loss, with serious consequences for ecosystem functioning [12]. Simultaneously, the combined effects of environmental factors have great potential to interact and indirectly or directly mediate soil moisture, affecting the main process of \(C\) cycling [165]. Similarly, analyses of the roles of other factors in grassland \(C\) cycling or ecosystem functions should not overlook the influence of drought. The contribution of each factor to grassland \(C\) cycling should be diagnosed and quantified, especially with regard to influencing the relationship between droughts and grassland \(C\) cycling in a realistic multi-factor world. Clearly, there has been much progress in the sophistication of both models and experiments. It is no longer considered acceptable to make projections about drought response without incorporating the likely effects from other factors, such as elevated CO\({}_{2}\) concentrations and global warming [169]. Doing so is significant in discussing how and why ecosystems have different sensitivities to chronic global change and pulsing droughts [12]. The multi-factor effects differ greatly from simple combinations of single-factor responses because the impacts of abrupt changes and alternative multi-factors induced nonlinear changes in the ecosystem [18]. Concurrent changes in multiple factors potentially trigger complex interactive impacts on ecosystem structure and functioning. For example, CO\({}_{2}\) enrichment restrains the effects of increasing temperature, precipitation, and available \(N\) on NPP annually in Californian grasslands [170].