Response Of Phenology ProduCtivity And Soil Carbon Stocks To Climate Change In Northern Chinese Steppes

Steppes are plant communities which mainly composed by hardy xerophytic perennial herb. Steppe ecosystem is one of the most widely distributed ecosystems worldwide. Northern Chinese steppes distribute widely and cover more than21%of entire territory of the country. The main steppes are divided into Temperate Meadow, Temperate Steppe and Temperate Desert Steppe which distributed in arid/semi-arid climatic zone; and Alpine Meadow, Alpine Steppe and Alpine Desert Steppe which distributed in Qinghai-Tibet Plateau climatic zone. In this study, we extracted the plant phenology characters basing on satellite data to explore the dynamics of plant phenology in Northern Chinese steppes and built a CENTURY-based modeling system to simulate the responses of net primary productivity and soil organic carbon to climate change in Northern Chinese steppes.In northern Chinese steppes, water use efficiency in temperate steppes regions is lower than that in alpine steppes regions because of the higher surface potential evapotranspiration. The net primary productivity (NPP) in Qinghai-Tibet Plateau is generally higher than that in temperate regions; alpine meadow has the highest capacity on ground biomass accumulation (500g DM m-2yr-1). Based on NDVI data, the results show that over the period from1982to2006, the growing season duration has lengthened by0.39days yr-1in northern Chinese steppes. The beginning of the growing season has advanced in spring by0.21days yr-1and the dormancy delayed in autumn by0.18days yr-1; alpine meadow has lengthened the most, with0.73days yr-1.We complied CENTURY from a site-based model into spatial model, and pro-processed the needed data and model parameters as spatial data. We tested the model using field survey and literature data. The consistency degree of above ground biomass and soil organic carbon density (SOCD) showed an ideal level (R2was0.83and0.56respectively). The result indicated that the average SOCD of northern Chinese steppes was4.6kg C m-2(0-20cm depth). The highest SOCD appeared in alpine meadow, with value of7.53kg C m-2; in temperate meadow and temperate steppe, the SOCD was5.31kg C m-2and3.59kg C m-2respectively; and in temperate desert steppe and alpine meadow steppe, SOCD was the lowest, with mean value of1.95kg C m-2and2.43kg C m-2. The total SOC stocks (0-20cm depth) was12.36Pg C (1Pg=1015g) and about70%was stored in alpine steppes region. The soil organic carbon (SOC) stock in alpine steppes region was2.29times than that in temperate steppes region.In order to evaluate the impact of climate warming on soil organic carbon of Chinese steppe, we designed a scenario for temperature increasing basing on actual climate data from1961to2009and assumed a4℃increase during the50years (The Canadian Climate Center and Geophysical Fluid Dynamics Laboratory High Scenario predicted that temperature increases in all global ecosystems ranging from3℃to6℃). A4℃increase in soil temperature led to a loss of4.7%and2.6%of soil organic carbon in the alpine meadow and alpine steppe; the decreasing rate was2.9%and2.3%in temperate meadow and temperate steppe respectively; but the same temperature increase led to a maximum loss of soil organic carbon in the alpine desert meadow and temperate desert steppe were only1.7%and0.3%respectively. This result indicates that alpine meadow will become the biggest potential carbon source in Chinese grasslands if climate warming continues to occur.Higher CO2concentration enhances plant photosynthetic efficiency and water use efficiency. The direct effect is to reduce carbon losses and tended to diminish the carbon loss driven by temperature increasing in northern Chinese steppes. In the temperature increasing scenario, if the CO2concentration elevation coupled with temperature increasing, the13%of soil carbon losses caused by4℃increasing could be offset. The offsetting rates of soil carbon loss happened obviously in temperate desert steppe and alpine desert steppe, with value of45%and75%; with value of31%and19%in temperate meadow and temperate steppe; but with value of9%and14%in alpine meadow steppe and alpine steppe.Overall, during the past25years, the growing season duration has lengthened in northern Chinese steppes but the lengthened range were diversity; the highest soil carbon was stored in alpine meadow, but it will become the biggest potential carbon source in northern Chinese steppes if climate warming continues to occur. Atmospheric CO2concentration increasing is the main cause for climate warming. However, it has positive effects on soil carbon sequestration. This study was only focus on the one-way responses of climate change on plant ecosystem. In the future, we will try to explore the interaction between plant ecosystem and atmosphere system, water systems, and human social systems and strive to evaluate the effects of global climate change on terrestrial ecosystem.