# Drought and Carbon Cycling of Grassland Ecosystems under Global Change: A Review
## 2 Drought and Grassland \(C\) Cycling under Global Change
### Elevated CO\({}_{2}\) Concentrations  
CO\({}_{2}\) fertilization effects may be affected when droughs occur [73]. An elevated CO\({}_{2}\) environment may also increase the water use efficiency of plants and thus soil moisture--due to the reduction in stomata conductance--and alleviate the impacts of drought [74]. Elevated CO\({}_{2}\) could mitigate the effects of drought on grassland net carbon uptake by increasing root growth and plant \(N\) uptake [75]. In fact, elevated CO\({}_{2}\) directly or indirectly affects plant water loss and may be crucial to understanding the combined effects of drought on the \(C\) cycling processes of ecosystems. The long-term effects of CO\({}_{2}\) on ecosystem functions are more likely to be indirect responses, such as changes in the biochemical cycle, soil moisture, and species structure, than direct responses through exchanges in production [76]. Elevated CO\({}_{2}\) not only increases long-term mean soil moisture [77], but also enhances microbial biomass and density [11]. To a certain extent, rising atmospheric CO\({}_{2}\) concentrations might decrease the vulnerability of grassland \(C\) to droughts [78]. Atmospheric CO\({}_{2}\) enrichment can enhance NPP and mitigate the negative effects of droughts on GPP and NEE [31; 79]. Likewise, elevated CO\({}_{2}\) concentrations alleviate the negative effects of droughts on soil respiration, principally due to the promotion of carbon assimilation, which increases the substrate supply for respiration in both roots and soil microorganisms [80]. Given a number of adverse environmental and edaphic conditions, the increased resource use efficiency of plant growth under high atmospheric CO\({}_{2}\) concentrations cannot prevent a decline in productivity and quality [78]. In one study, elevated atmospheric CO\({}_{2}\) reduced the sensitivity of grassland ecosystems to drought and increased grassland productivity by \(\sim\)5%-15%, depending on water and nutrient availability [81; 82]. However, CO\({}_{2}\) fertilization decreases when water and \(N\) are limited. In fact, these positive effects are unlikely to offset the negative impacts of high temperature changes and decreased summer rainfall, which would lead to more frequent and intense droughts [83]. Consequently, beneficial CO\({}_{2}\) fertilization effects are suppressed, and water use efficiency is not enough to compensate for the negative effects of droughts [84]. As the intensity and duration of droughts increasingly result in stomatal closure, the ecosystem can become CO\({}_{2}\) limited, which may stimulate \(C\) starvation and hydraulic failure [85; 86]. Concurrently, droughts can also lead to increasing GHG (Green House Gas) emissions, limiting the vegetation \(N\) supply, and change the amount of fixed carbon [87]. The effects of concurrent elevated CO\({}_{2}\) levels and droughts on a plant's water use efficiency are intertwined. As global climate models predict rising temperatures, it is important to acknowledge that CO\({}_{2}\) concentrations will interact with the change in precipitation patterns, thereby affecting \(C\) cycling. In dry soils, elevated CO\({}_{2}\) concentrations coincide with or even contribute to drier conditions, resulting in negative soil respiration responses to temperatures [88]. Elevated CO\({}_{2}\) concentrations and extended droughts show a positive interaction, and elevated CO\({}_{2}\) can reduce the effects of drought on soil respiration [80]. In contrast, elevated CO\({}_{2}\) concentrations restrain diversity, but have little effect on the relative abundance or the production of a community [89]. Thus, those effects indicate the importance of a multi-factor experimental approach to understanding an ecosystem's response to droughts [18]. Indeed, the interactive effects of multiple factors on grassland \(C\) cycling under different drought scenarios are lacking in the literature, especially studies discussing underlying mechanisms.