Review of drought impacts on carbon cycling in grassland ecosystems

Grasslands play a key role in both carbon and water cycles. In semi-arid and arid grassland areas, the frequency and intensity of droughts are increasing. However, the influence of a drought on grassland carbon cycling is still unclear. In this paper, the relationship between drought and grassland carbon cycling is described from the perspective of drought intensity, frequency, duration, and timing. Based on a large amount of literature, we determined that drought is one of the most prominent threats to grassland carbon cycling, although the impacts of different drought conditions are uncertain. The effects of a drought on grassland carbon cycling are more or less altered by drought-induced disturbances, whether individually or in combination. Additionally, a new conceptual model is proposed to better explain the mechanism of droughts on grassland carbon cycling. At present, evaluations of the effects of droughts on grassland carbon cycling are mainly qualitative. A data fusion model is indispensable for evaluating the fate of carbon cycling in a sustainable grassland system facing global change. In the future, multi-source data and models, based on the development of single and multiple disturbance experiments at the ecosystem level, can be utilized to systematically evaluate drought impacts on grassland carbon cycling at different timescales. Furthermore, more advanced models should be developed to address extreme drought events and their consequences on energy, water, and carbon cycling.