Changes Of Soil Organic Carbon And Total Nitrogen Of Croplands In China And The Net Carbon Effiency Of Rice And Wheat Crops Determined By Nitrogen

The observed and projected increase in emission of greenhouse gases, with attendant effects on global warming, have raised interests in identifying mitigation options. Terrestrial C sequestration involves capture of atmospheric C through photosynthesis and storage in biota and soil. As a significant contributor to the regional carbon budgets, agro-ecosystem plays an important role in terrestrial ecosystem carbon cycling. In comparison with natural ecosystems of forest and grassland, agro-ecosystem is much complex due to human being activities, particularly the application of synthetic N fertilizer which was considered as an effective practice to promote crop production, also augments C inputs to the soil and hence, often increases soil organic carbon (SOC).It has been well recognized that the addition of N fertilizer promotes crop tissue N and the tissue N changes as crop growing. Numerous studies have demonstrated that plant photosynthetic capacity correlated strongly with leaf N concentration and a linear relationship existed between plant respiration and tissue N for forest. However, crop respiration regulated by tissue N is still far from being understood. Though previous studies indicated that the augmentative effect of fertilization on crop productivity would decrease as the fertilizer application exceeds a certain rate, all these results came from the evaluation of fertilizer application on photosynthesis in physiological views or on production benefit in economic views. Moreover, the influence of N application on crop respiration and a synthetic assessment on crop carbon sequestration with different rates of fertilizer N application is not very clear.The objectives of this study are, 1) to evaluate the changes in SOC at national scale over the last two decades by pooling and analyzing a mass of data sets from published papers towards providing a scientific basis in making soil management policy, enhancing soil C sequestration and reducing soil C loss; 2) to address the changes of total nitrogen...