Chronic and intense droughts differentially influence grassland carbon-nutrient dynamics along a natural aridity gradient

Abstract

Grasslands are expected to experience both long-term chronic reductions in precipitation as well as increased frequency of short-term intense droughts. However, few studies have assessed how these two types of drought differentially alter carbon-nutrient dynamics of grassland vegetation and soil on broad spatial and temporal scales. We conducted a two-year drought experiment in three types of grasslands along a natural aridity gradient in northern China. In each grassland, we removed ~ 50% of annual rainfall using two methods—chronic drought (66% reduction of each rainfall event for four months) and intense drought (100% removal of rainfall for two months). This allowed us to compare the effects of these drought characteristics on carbon and nutrient content of both vegetation and soil. Drought largely led to decreased carbon and nutrient pools, with vegetation concentrations being less responsive than pools likely due to decreased plant biomass. These responses depended on drought type, with no clear directional pattern of intense droughts having a greater effect than chronic drought. Sensitivity of biogeochemical responses to drought treatments decreased with increased aridity, likely due to the high abundance of drought-tolerant species in more xeric grasslands. Overall, ecosystem biogeochemical responses to manipulative drought did not match trends observed along the natural aridity gradient. The sensitivity of carbon-nutrient dynamics of plant and soil strongly depends on drought type as well as local climate and species composition. Such differential drought responses highlight the challenge of predicting ecosystem responses to climate change over large spatial scales.