| Global climate change is expected to increase the frequency of extreme drought(ED),resulting in a temporal reduction in water availability across ecosystems that span broad aridity gradients.The allocation of carbon to plant biomass and soil is a fundamental ecosystem process that varies spatially and temporally with water availability which have potentially prolonged effects on terrestrial carbon cycling.In natural grasslands worldwide,plant and soil carbon(C)dynamics are the key component of the ecosystem C cycle which generally disrupted due to extreme climatic event such as ED.Also,‘type’ of drought such as long-term ‘chronic’ as well short-term ‘intense’ has variable effect on grassland plant and soil C dynamics.Yet,how ED and drought type affects the grassland plant and soil C dynamics along natural aridity gradients remains poorly understood.To elucidate these effects,we conducted a 4-year(2015-2018)precipitation manipulation experiment(The EDGE experiment)where precipitation was manipulated using two methods – chronic drought(66% reduction during growing season for four months-May to August)and intense drought(100% reduction for two months-June to July)across 6 grasslands that spanning desert,typical and meadow steppe in northern China.Thus,the main scientific questions of this thesis were: 1)How plant vegetation C pools(e.g.,aboveground,belowground and litter)response to experimental drought along natural precipitation and aridity gradient?,2)How drought type(e.g.,Intense drought and chronic drought)impact on soil C along natural precipitation and aridity gradient?,and 3)How the sensitivity of soil C to drought type would depends on grasslands vegetation types(e.g.,desert,typical and meadow)and net primary productivity(e.g.,ANPP and BNPP)?On average,extreme drought significantly decreased above-ground biomass carbon(35.02%)and litter carbon(28.07%),but increased below-ground biomass carbon(5.35%)at the 6 experimental sites,resulting in a decrease in grassland vegetation carbon storage(about 10%)overall.Decrease in aboveground biomass carbon and litter carbon caused by extreme drought decreased with increasing precipitation.In contrast,extreme drought increased more belowground biomass carbon in drier grasslands.Soil organic carbon decreased by an average of 1.8-7.5% due to extreme drought at the six experimental sites.Chronic drought treatment had a higher negative impact on soil organic carbon than intense drought treatment.In the desert steppe with low natural rainfall,the decrease of soil organic carbon was the largest,while the decrease in soil organic carbon was smaller in the meadow steppe and typical steppe with high natural rainfall.Under the two extreme drought types,the response of soil organic carbon was significantly negatively correlated with ANPP and significantly positively correlated with BNPP,and was affected by natural rainfall and grassland types.Overall,this study shows that extreme drought reduces the plant and soil carbon pools in Inner Mongolia grasslands,and the degree of extreme drought effects is affected by the type of extreme drought,the response of plant productivity,and natural rainfall conditions.Our findings contribute to a deeper understanding of the impact of increasingly frequent extreme droughts on grassland plant and soil carbon dynamics,which could serve as a valuable reference for predicting the response to climate change in the carbon cycle of grassland ecosystems in Inner Mongolia. |
