Quantification Of Waste Nitrogen In Food Chain In Changshu City And Its Regulation Mechanism

Nitrogen is an indispensable nutrient for the growth of plants and animals.It is also one of the key elements controlling the structure and function of terrestrial,freshwater and marine ecosystems.The study of the nitrogen amount in the food chain system "crop-livestock-household" could safeguard human health and lay foundation of food production.With the rapid improvement of living standards and fast development of economy,great change has taken place in the structure and quantity of people’s food consumption.The proportion of animal-derived food demand has just increased.Due to the fact that the amount of nitrogen input in the food chain system has been increasing,excessive nitrogen input has become the biggest environmental problem today.In this paper,we focused on Changshu county in the lower reaches of the Yangtze River as the research target area.The material flow analysis and mass balance method combined with field research were used to construct a food chain production-consumption nitrogen flow model,for analyzing quantitatively the nitrogen in the crop-livestock-household system.The waste nitrogen associated with its environmental effects and food nitrogen footprint were calculated and compared.According to the characteristics of nitrogen flow in the system,some control measures were put forward to improve regional environmental quality.Therefore,the social-economic-environmental sustainable development goals would be achieved.The main research results are as follows:(1)During 2000-2016,the total inputs of nitrogen in the food chain of Changshu area decreased from 4.55×104 t N yr-1 to 3.3×104 t N yr-1.The crop production subsystem has become the main nitrogen input source in food chain(77.84%).This maybe caused by the ever-increasing urbanization rate and the shrinkage of farmland.(2)Compared with 2000,the amount of waste nitrogen produced by the "crop-livestock-household" system in 2016 decreased by 31.17%.During this period,the poultry breeding subsystem contributed dominantly to the total waste nitrogen,which was 36.84%,followed by the household consumption subsystem of 35.90%,and 27.25%from the crop production subsystem.The waste nitrogen is mainly recycled through returning to the field and as feed.The nitrogen circulation rate is low(18.46%-21.85%).(3)During 2010-2016,food nitrogen footprint of urban residents in Changshu county was 14.68～20.00 kg N person-1 yr-1(the average amount was 17.51±1.53 kg N person-1 yr-1)and the per capita food nitrogen footprint of rural residents was 17.58～28.46 kg N persorn-1 yr-1(the average amount was 20.71±3.01 kg N person-1 yr-1)The food nitrogen footprint of urban residents was mainly caused by animal-derived food(66.42%).While the rural residents’ food nitrogen footprint was changed mainly from the plant-derived food in 2000(51.90%)to be animal-derived food in 2015(62.40%).The results of correlation analysis of economic factors demonstrate that per capita GDP,urbanization rate and per capita disposable income are the main factors affecting the per capita food nitrogen footprint of urban and rural residents in Changshu county.(4)The main sources of environmental nitrogen load include chemical fertilizer application,mannure,straw and kitchen waste.The average nitrogen load per unit area in crop production subsystem was 204.48±13.12 kg N hm-2,accounting for the largest proportion of 63.16%of the total emissions.The average emission per unit area of livestock breeding subsystem was 64.53±8.08 kg N hm-2,accounting for 19.93%of the total emissions.The average nitrogen load per capita household consumption subsystem is 3.69±0.11 kg N person-1.If all the waste nitrogen are utilized,these will greatly alleviate the power grid and municipal waste treatment pressure in the Changshu area.The waste nitrogen utilization may recycle effectively the resources,and conform to the concept of building a resource-saving and environment-friendly society.The measures are benifit for the sustainable development in society.(5)According to the above results,we proposed four control methods for regulating nitrogen flow in the food chain of Changshu county in this paper:① to improve nitrogen utilization rate and reduce nitrogen emissions in food chain.②to optimize farmland planting and food consumption structure,formulate policies and regulations,and reduce resource and environmental pressures.③ to accelerate the process of industrialization of agricultural waste resources utilization,and vigorously develop circular economy.④to strengthen the infrastructure construction of waste utilization,and encourage research on innovation of technology in transforming the wastes into resources.