| Increasing nitrogen (N) fertilizer application has greatly contributed to the long term food security in China during the last four decades but also contributed to rapidly increasing N losses to groundwater, surface waters and air. However, the quantitative insight and relationship between food N demand, N input and N losses to environment, the mechanism of nitrogne flow in the food chain, the pathways and effect of different nitrogen managements are still lacking. Here, we present N flow budget, the feature of nitrogen flow from food chain perspective and the effect on envrioment at national and regional scale of year 2005, using the model NUFER (NUtrient flows in Food chains, Environment and Resources use). The nitrogen flow mechanism and managemtent strategies in the Food Chain of China should be expolored further. The main results were as follows:(1) It is the first comprehensive overview of N balances, losses, use efficiencies and N flow mechanism of the food chain at national and regional scale in China. NUFER also allows to estimat changes in N flow budget due to different N management strategies.(2) Total amount of 'new' N put to the food chain was 48.8 Tg in 2005. Only 4.4 Tg reached households as food nutrient. Average N use efficiencies in crop production, animal production and crop and animal production and food processing were 26.4,11.1 and 8.9%, respectively. Most of the applied N was lost to the environment, in which 23 Tg N to atmosphere and 20 Tg to waters.(3) The surplus and deficiency rate of fertilizer,feed,plant food and animal food was 33%,-120%,38%,65%, respectively in 2005 in Huang-Huai-Hai. The driving force of nutrient flow in local food chain is the potential nutrient. But the factor of population,urbanization,arable land area,GDP,transport distance,transport price,market price and policy etc also have influence on it. In 2005, the area of Huang-Huai-Hai is the source of nitrogen of fertilizer and food, and the stock of nitrogen of feed. But in Beijing, nitrogen of fertilizer, food and feed all is the stock. The average cropland load of nitrogen which comes from other provinces is 872 kg hm-2, which still has a great risk to environment even though all this nitrogen has been returned to the cropland. So the collaborative management of nutrient should be carried on in Beijing-Tianjin metropolis circle immediately. (4) Mean N losses from crop and animal system amounted to 327 kg hm"2 agricultural land in 2005. On average 66% of the N losses originated from crop production and 34% from animal production systems, but regional variations were large. Leaching to groundwater and surface waters was the main loss pathway, ranging from 33 to 300 kg N hm-2 among provinces. Volatilization of NH3 was the 2nd main loss pathway (range from 29 to 215 kg N hm-2). Mean emissions of N2O ranged from 1 to 6 kg N hm-2. Highest N losses were estimated for provinces in south east and in the North China Plain with highly intensive agricultural systems, In the balanced fertilization scenario, mean N losses decreased by 34% relative to the baseline scenario, but with large differences between provinces (range from-35% to 57%). In scenario 2 (improved manure management), mean N losses by another 7%. In scenario 3, we assumed a 20% increase in crop yields, as a result of higher-yielding varieties and improved water, nutrient and pest management.(5) With the growth of GDP and food consumption, the amount of fertilizer, feed, the food nitrogen consumption per capita are increasing dramatically, which in 2005 were 2.1,2.2 and 1.3 times of that in 1980 respectively, and the resources cost to support one Chinese is increasing year by year. With the increasing of food demand, the nitrogen amount of pool and the flow in the food chain in China grew remarkably. In 2005, the amount of nitrogen flow of crop production, animal production and household consumption were 43.5,22.6 and 4.4 Tg and that in 1980 were only 21.0, 7.5 and 3.1 Tg, respectively. However, the nitrogen use efficiency of food in china was only 9% in 2005 and the waste recycling rate was decreased. Meanwhile, the remainder of nitrogen to a great part lost to the environment, which means that 42.9Tg N lost to atmosphere and water in 2005, which were 2.4 times of that in 1980. Hence, food chain system has become the channel of nitrogen activated.(6) With composition forces of the pull, impetus, sanction and control forces (scenario 5), the food N consumption woule be 6.5 Tg and increasing 48% compared with that in 2005, based on the changing of populations, food structure and urbanization etc. But there are limited resources, which can not surport so many "New N" input, and environment effect as well. Hence, nitrogen use efficency should be increased under the control forces and impetus. Extending the implementation of nitrogen management, "New N" will be increasing 12%, losses to environment increasing 7%, and nitrogen use efficiency will increase from 9% to 16%.
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