Abstract
<p> 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 the foundation for food production. In this paper, we focused on Jiangsu Province 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. During 2000~2016, the total inputs of nitrogen in the food chain of Jiangsu Province area decreased from 3.38×10<sup>6</sup> t N yr<sup>-1</sup> to 3.15×10<sup>6</sup> t N yr<sup>-1</sup>. The crop production subsystem has become the main nitrogen input source in the food chain (77.84%). This may be caused by the ever-increasing urbanization rate and the shrinkage of farmland.</p><p>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%). The main sources of environmental nitrogen load include chemical fertilizer application, manure, straw and kitchen waste. 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 benefits for sustainable development in society.</p>
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