| This study is the continuation of "Nutrient Cycling as a Measure of Agroecosystem Sustainability" (Grant DEB-9303261) of American state natural science foundation (NSF), and main content of "Countermeasures of Agricultural Non-point Source Pollution Control in Tai lake Area" which is an important application basis study project of agricultural ministry in 1996. Adopting macro (survey data and statistical data) and micro data combined methods, the dissertation, studies the relationship between nitrogen application amount and yield, nitrogen efficiency and environment loss from province, region, city, county, and township different level, assesses rationality of nitrogen, phosphorus, potassium fertilizer application amount of paddy field ecosystem in Tai lake area, discusses ecological and economic nitrogen application amount combining with field experiment results, puts forward the suggestions of nitrogen sustainable management in Tai lake area. Following are main contents and conclusions.1. Results of Field experiment under the same conditions except the difference of nitrogen application amount show that nitrogen fertilizer income reduces and nitrogen absorption amount in straw increases as nitrogen application amount increases. Rice yield increases, but its absolutely increased amount becomes smaller and smaller, at the mean time, average yield, marginal yield, production elastic co-efficiency and income and investment ration decrease with the increase of nitrogen application amount; Rice and wheat grain nitrogen current season efficiency increases first, decreases lately, but straw nitrogen current efficiency always increases with the increase of nitrogen application amount, which shows that there is a serious nitrogen extravagant absorption phenomena with the nitrogen application amount increase. According to traditional economy and environmental economy, ecological and economical application amounts are 281.9 and 336.3 kg/ha respectively.2. Survey and field experiment results show that nitrogen over application not only causes gradual accumulation of nitrogen in soil, but also increases nitrogen loss in water. Compared with conventional irrigation, rice yield, nitrogen fertilizer current season efficiency can be increased by 4.1% and 3% respectively, and nitrogen leaching amount can be decreased by 47.98% with water saving irrigation under the same nitrogen application amount (264.75 kg/ha), which shows that water saving irrigation has obvious economic and ecological benefit.3. Polluting sources of water pollution and Tailake eutrophication in Tai Lake area, Jiangsu province are field lost nitrogen in water, human and animal waste, carefully cultivated fishpond, life waste water, industry wastewater drainage into water. Results show there is 15787 ton of nitrogen in water from field in Tai Lake area in 1997. Pollutant equivalent standard discharging amount analysis results show that phosphorus is main and nitrogen, CODcr are second threat to water environment in view of human and animal manure urine, carefully cultivated fishpond, life waste water. There are 16820 tons of CODcr, 7241 tons of TP, 10455 tons of TN discharged into water from animal and poultry manure urine in 1997; 89385, 2239, 8939 from life polluted water; 74551, 1969, 11498 from human manure urine; 2758.5,407.3, 3739.7 from carefully cultivated fishpond. Nitrogen proportions in water from field, animal poultry manure urine, life polluted water, human manure urine, carefully cultivated fishpond and industry waste water are 24%, 16%, 14%, 18%, 6% and 22% respectively. Pollutants from crop planting, husbandry and fishery have become important water polluting source and is important reason of Tai Lake eutrophication and nitrogen in water from field, animal manure @urine, and carefully cultivated fish pond amounts to 46%.4. Comparison analysis results of nutrient balance with change of soil nutrient content in Tai lake area of Jiangsu province show that nitrogen fertilizer is over applied, potassium inadequatedly applied in this area. Nitrogen fertilizer application amount should be reduced by 17%, potassium and phosphorus fertilizer application amount should be increased by 70% and 2% respectively, N:P2O5:K2O should be 1:0.23:0.47 in this area according to nutrient balance analysis. From 1990 to 1997, there are 650.1, 558.9, 741.3 kg/ha of surplus nitrogen, 658.7, 547.5, 769.7 kg/ha inadequate potassium in Tailake area, Wuxi city, Changzhou city. In the view of phosphorus, it is under applied in Tailake area and Changzhou city and over applied in Wuxi city. From 1995-1998, there are 423.6 kg/ha nitrogen, 22.1 kg/ha phosphorus over applied, 197.5 kg/ha potassium under applied in Wuxian city (county level). There are more than 85% townships whose nitrogen fertilizer is over applied with more than 300 kg/ha, more than 70% township whose phosphorus fertilizer is under applied, closely 80% township whose potassium fertilizer is under applied with more than 100 kg/ha in 15 townships of wuxian city from 1995-1998. From the change of soil nutrient content, organic matter, total nitrogen, efficient phosphorus in Wuxi soil are increased by 19%, 27% and 16% and effective potassium in soil is decreased by 22% in recent 10 years in this city.5. Nitrogen fertilizer efficiency of Jiangsu province is lower than average level of whole country. Rice nitrogen fertilizer current season efficiency is from 14.4% to 28.8%, averaging 19.9% in 6 agricultural areas in Jiangsu province, 17.2% in Tailake area; Wheat from 16.2% to 25.2%, averaging 19.5% in Jiangsu, 25.2% in Tailake area. Averaged rice nitrogen fertilizer accumulated efficiency is 43% in whole province, 47% in Tailake area; Wheat 41% in whole province, 47% in Tailake area. There is no significant difference of nitrogen current efficiency, nitrogen accumulated efficiency between rice or wheat bumper harvest and ordinary field. Rice current season nitrogen efficiencies of bumper harvest field and ordinary field are 21.4% and 18.4% respectively in whole province, wheat 19.2% and 20.2%; Rice accumulated nitrogen efficiencies are 41% and 44%, wheat 40% and 43% respectively. There are significant difference of rice, insignificant difference of wheat current season nitrogen efficiency and accumulated nitrogen efficiency in different areas of Jiangsu province.6. Fertilizer application amount in Jiangsu province is higher than average amount of whole country and whole world. The proportion of organic nitrogen in total nitrogen application amount of Jiangsu province is equal to that of China. Proportion of compound and urea fertilizer is higher than average level of China; There is significant or very significantdifference of nitrogen and phosphorus application amount between rice or wheat bumper harvest field and ordinary field. There is significant difference of potassium fertilizer application amount of rice, insignificant difference of wheat between bumper harvest field and ordinary field; Except rice nitrogen application amount, there is very significant difference of nitrogen, phosphorus, potassium fertilizer application amount in different areas of Jiangsu province. (l)Nitrogen, phosphorus, and potassium fertilizer application amounts in Jiangsu province paddy field in 1995 are 663, 221, 373 kg/ha, which is 3, 2, 4 times as much as those of same unit cultivated area in China, and 10, 9, 23 times as much as chemical nitrogen, phosphorus and potassium fertilizer application amount in the world. The proportions of organic nitrogen fertilizer in total nitrogen are 19% to rice, 28% to wheat in whole Jiangsu province. Pig manure amounts to 50% of total nitrogen fertilizer applied in Jiangsu province. Besides pig manure, crop residue, human manure, big animal manure is also main organic nitrogen fertilizer in whole country. The proportions of nitrogen compound fertilizer in Jiangsu and whole country are 10% and 4% respectively and those of urea are 45% and 43%. (2)Nitrogen, phosphorus, potassium fertilizer application amounts in bumper harvest and ordinary rice field are 372, 105,185; 327,77.6,140 kg/ha respectively, and those in wheat field are 350, 146, 237; 283, 114, 180 kg/ha respectively. (3)Compared with whole Jiangsu province and other agricultural areas, nitrogen, phosphorus, potassium fertilizer application amounts in Tailake area are lowest, organic nitrogen fertilizer proportion in total nitrogen in less than half of the Jiangsu average figure and chemical nitrogen constitution is worse than Jiangsu average level. Nitrogen, phosphorus and potassium fertilizer application amounts in Tailake area are 575, 108, 173 kg/ha respectively and the proportion of urea, ammonia carbonate compound fertilizer in rice season of whole province and in Tailake area are 45.77%, 44.47%, 9.7% and 39.47%, 48.72% and 11.81% respectively.Low nitrogen fertilizer efficiency is caused by nitrogen over application, the unreasonable proportion of N, P2O5, K2O, et.al., which shows high nitrogen loss in environment and is an important obstacle in agricultural sustainable development. In order to realize agriculture sustainable development in Tai lake area and in Jiangsu province, countermeasures of lowering nitrogen fertilizer amount, bettering fertilizer structure, soil r'ecomendation, water saving irrigation should be adopted.Some content of the study has been written into four articles published in Chinese famous magazines; The rest has been written into six articles which will be published in magazines home and abroad.
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