Carbon And Nitrogen Balance Modeling,Coupling And Policy Evaluation In China's Agro-ecosystem

As one of the basic industries of China,the primary objective and task of agriculture has long been developing the agriculture economy to meet the ever growing demand for food.Compounding this challenge,agriculture is now a dominant source of many environmental threats,including greenhouse gas emission,water pollution and degradation of land.Under the dual pressures of food security and environmental pollution,modern agriculture is facing great challenges in production engineering and development model.As agro-ecosystem is an open and complex system,efficient techniques of examining chemical element flows,identifying pollution avenues and sources as well as quantifying emissions of pollutants are the major scientific and technical issues to be addressed.Therefore,in order to analyze the properties of China's agro-ecosystem and reveal the disturbance of agricultural activities on the natural recycling of carbon and nitrogen,this study aimed to establish carbon and nitrogen balance models adapted to the characteristics of China's agriculture.Besides,the models can also provide reference and technical support for assessing various sources of agricultural pollution and developing low-pollution agriculture.According to the material balance theory,this study established national-scaled carbon and nitrogen balance models for China's agro-ecosystem based on the material flow method and the main carbon/nitrogen cycling processes.With the models,this study simulated inputs and outputs of carbon and nitrogen in China's agro-ecosystem in 1980,1990,2000,2010 and 2013,identified the key pathways and mechanisms regulating carbon and nitrogen cycles,investigated primary agricultural pollution sources and intensities,and analyzed characteristics of long-term dynamic variation of agroecosystem carbon and nitrogen cycles.In the meantime,with available agricultural data and model estimates,this study also sought to identify the coupling relationships of carbon and nitrogen in farmland ecosystem.On this basis,this study further analyzed the policies of China's agricultural pollution control and discussed the surplus/deficit of carbon and nitrogen as well as the pollutant emissions under different designed scenarios so as to evaluate the effectiveness of such policies.Results showed that: China's agro-ecosystem represented as carbon deficit and nitrogen surplus in all five selected years;the main input source of carbon was by photosynthesis of plants while chemical fertilizer brought most nitrogen into the agroecosystem;the main ways of carbon emission were plant respiration and soil respiration,as more than 80% of agricultural carbon was emitted into the atmospheric environment through respiration;ammonification,nitrification/denitrification and groundwater leaching were the main output ways of nitrogen.The results of coupling research revealed the positive correlations between atmosphere/soil carbon and nitrogen input amounts as well as output amounts of farmland ecosystem,and the negative correlations between carbon fixation by photosynthesis/carbon emission by respiration per unit of crop area and the carbon-nitrogen ratio in soil plough layer respectively.Quantitative simulation and analysis of selected policies shows that avoiding burning straw in the open air and using straw as fuel can directly control carbon and nitrogen emissions;raising the proportion of straw and livestock manure that is returned to the field as well as using straw as feed rather than as biofuel can effectively reduce the net carbon deficit of agro-ecosystem.