Drought Impacts on Soil Organic Carbon in Grasslands Depend on Climate, Drought Type, and Net Primary Productivity

Grasslands are expected to experience both long-term ‘chronic droughts’ as well as short-term ‘intense droughts’ due to climate change. However, very few studies have assessed how these two types of droughts differentially influence grassland soil organic carbon (SOC) storage. We explored this effect using a 4-year (2015-2018) precipitation manipulation experiment in which drought was imposed in two ways – chronic drought (66% reduction during the growing season) and intense drought (100% reduction for two months) – across six grassland sites spanning desert steppe, typical steppe and meadow steppe in northern China. Our results revealed that extreme drought decreased SOC and the loss was higher in arid grasslands (desert steppe and typical steppe). Chronic drought decreased SOC more than intense, with the effect again being strongest in arid grasslands. Furthermore, the response of SOC to drought was influenced by the response of net primary productivity. Specifically, the response of SOC was negatively related to drought sensitivity of above-ground net primary productivity (ANPP) but positively related to drought sensitivity of belowground NPP (BNPP). Overall, our results suggest that shifts in grassland SOC storage with future drought will depend on the type of drought as well as the productivity responses and local climatic conditions. The differential drought impacts described here may facilitate predictions of climate change impacts on ecosystem carbon cycling.