| Taihu Lake region is one of the most famous agriculture areas of high yield in China; its rice-wheat double cropping system has a long history. In recent years, the concentration of N and P in water of Taihu Lake is on the high side, the eutrophication of surface waters is increasing seriously, non-point source pollution from agriculture have attracted considerable attention. Nitrogen application rate in rice-wheat double cropping system of Taihu Lake district was always higher. Owing to the long-term excessive use of nitrogen, this not only results in the lower nitrogen use efficiency, but also increases nitrogen surplus in soil-crop system, causing higher content of background nitrogen in soil. So the utilization and balance characteristic of nitrogen in soil-crop system, the aim of higher crops yield and the improvement of environment, the appropriate content of background nitrogen in soil and nitrogen application rate were studied. These researches had great theoretical and practical significance to the scientific management of nitrogen nutrient in rice-wheat double cropping system of Taihu Lake district, and to the decrease of non-point nitrogen pollution to our environment.This research based on the current situation of higher content of background soil nitrogen in Taihu Lake region, by doing some experiments about the location of nitrogen application and the exhaustion of nitrogen, the efficiency of nitrogen application rates and content of background soil nitrogen on crop yield, the accumulation and transmission of nitrogen in soil profile, nitrogen use efficiency and so on were studied. With the appropriate amount of nitrogen application, with the appropriate nitrogen application rate, the research aimed at keeping the harmonions relationship among the condition of earth, the protection of environment, the higher yield of rice and wheat and the higher use efficiency of nitrogen. From 2007 to 2009 the experiments were carried out. In the wheat season of year 2007, the researchers took samples of soil profile with the layer of 0～20cm,20～40cm,40～60cm,60～80cm and 80～100cm from the representative farmland in counties of Wujiang, Liyang, Yixing, Wujin, Changshu, Xiangcheng, and then analysed the nutrient characteristic of soil profile and its adsorption characteristic. On this basis, from 2007 to 2009, the located experiments of nitrogen application and nitrogen exhaustion in the rotation of rice and wheat about five crop seasons which were rice-wheat-rice-wheat-rice from first season to fifth season and sixth wheat was growing in three years were carried out in the experimental station of Farming and Forestry Bureau of Wujiang county, Suzhou city, Jiangsu province, China. There were the areas of nitrogen exhaustion (without nitrogen treatments) and areas of nitrogen application (with five nitrogen application rates). According to the amount of pure nitrogen application in rice-wheat system in one year, the treatments of different nitrogen application rate were 0, 175, 350, 525, 700 and 875 kg/ha, the nitrogen fertilizer was divided into 57% for rice and 43% for wheat, so the nitrogen application rate for rice were 0, 100, 200, 300, 400 and 500 kg/ha, and for wheat were 0, 75, 150, 225, 300 and 375 kg/ha, the nitrogen fertilizer was all urea. The nitrogen application of rice was that basal fertilizer, tillering fertilizer, ear fertilizer in four leaves from top panicle stage and two leaves from top panicle stage were applied equally for four times. The use of nitrogen for wheat was that 60% in basal fertilizer and 40% in jointing stage by broadcasting. The experimental process was that first rice experiment was stated in 2007, an anniversary cropping of rice and wheat was a unit. According to the design, in 2007, there were five nitrogen application areas and the others were no nitrogen application areas. After the harvest of rice, the wheat sowed in the corresponding areas for the same experimental purposes. In 2008, to the nitrogen application areas, the experiments of 2008 were continued on the base of 2007, meanwhile, five studies about the quantity of nitrogen application were carried out on these areas, and also some areas were continuously kept to do experiments without nitrogen. The experiments of 2009 were carried out in the same ways like in 2008. The areas of nitrogen exhaustion were programmed before the year of 2007. At the same time, by using the soil Lysimeter in the experimental base of Yangzhou university, the nitrogen behavior and quantity characteristics in soil-crop system of rice-wheat rotation, such as the dynamic changes of NH4+-N concentration in surface water, ammonia volatilization and the leaching of nitrogen in soil after applying nitrogen fertilizer in rice season, the accumulation of nitrogen of crop and the dynamic changes of inorganic nitrogen in soil profile in wheat season were studied. The main results were as follows.First, the nitrogen content of cultivated land in each county around Taihu Lake Region was higher than the average standard of Jiangsu province. In the surface soil, the content of total nitrogen and available nitrogen were the highest, but they decreased when the depth of soil increased. To different soil's different layers, the amount of NH4+-N absorption was,different, the absorption ability of clay soil was higher than sandy soil, and the NH4+-N absorption ability of subsoil was higher than surface soil. With the increase of NH4+-N concentration, the NH4+-N absorption ability of soil increased too.Second, in rice season, the amount of ammonia volatilization losses depended on the density of NH4+-N in surface water and also related to the nitrogen application rate, the amount of ammonia volatilization losses and the density of NH4+-N in surface water all increased as the nitrogen application rate increased. The quantity turned to the peak in 1~3 days after applying the nitrogen, and the highest density of NH4+-N and amounts of ammonia volatilization losses appeared at the same time, one week after applying the nitrogen was the key time to reduce the nitrogen loss of ammonia volatilization and runoff. The loss of ammonia volatilization in clay soil was lower than that in sandy soil, the total loss of ammonia volatilization from clay soil was 10.49~87.06 kg/ha, and the proportion to nitrogen application rate was 10.92%~21.76%; the total loss of ammonia volatilization from sandy soil was 11.32~102.43 kg/ha, and the proportion to nitrogen application rate was 11.32%~25.61%. After applying different stages of nitrogen fertilizers, the ammonia volatilization losses were that the tillering stage > four leaves from top panicle stage > basal stage > two leaves from top panicle stage, and also we found that when the nitrogen application rate about 300 kg/ha in rice season, the amount of ammonia volatilization was higher than that about 200 kg/ha. Nitrogen leaching was mostly brought by flooding the farmland, NO3--N was the main form of nitrogen in leaching waters, NH4+-N was very little. The leaching of NO3--N in sandy soil was more than that in clay soil, the leaching of NO3--N which with higher nitrogen application was more than that with lower nitrogen application, and the leaching of NO3--N in the soil which owns higher leaching ability was more than that owns lower leaching ability.Third, different nitrogen application rates in soil-crop system were applied continuously and applied after one year and two years'exhaustion, the response of rice yield to nitrogen application rate was obvious different, it presented that the domino effect of content of background nitrogen in soil on rice yield was distinctness. When the amount of nitrogen application were 100 kg/ha and 200 kg/ha, the rice yield decreased in three years, it showed that soil nitrogen exhaustion made the content of background nitrogen decrease, we couldn't get high rice yield. When the nitrogen application rate was 300 kg/ha, the treatments of nitrogen applied continuously and applied after one year and two years'exhaustion all got high yield. But when the nitrogen application rate went even higher than that point, the rice plants lodged and yield decreased. So the nitrogen application rate in which rice got high yield was 300 kg/ha in a season and 525 kg/ha in one year, it would keep the content of background nitrogen which could ensure the high yield of rice. The content of background nitrogen in soil had some effect on nitrogen absorption accumulation of rice. The use efficiency of nitrogen fertilizer decreased when the nitrogen application rate increased, it were not related to the content of background nitrogen in the soil.Forth, the inorganic nitrogen in soil profile (INISP) in wheat season was not only related to the nitrogen application rate (NAR), but also to the background nitrogen level in the soil. The INISP mostly concentrated at the depth of 0-30 cm of the soil and increased as the NAR increased at different growth stages of wheat, the inorganic nitrogen at the depth of 30~100 cm of the soil had an obvious response to the NAR when the amount was in the range of 300 kg/ha to 375 kg/ha, and it had obvious eluviations effect after jointing stage; when the NAR was in the range of 300 kg/ha to 375 kg/ha, the remains of INISP was always higher, compared with the sowing stage, the INISP in maturity stage increased only when was given the treatment that the NAR was 375 kg/ha, otherwise, it all decreased when the NAR was smaller than 300 kg/ha. In wheat-soil system, the apparent losses of nitrogen increased as the NAR increased. When the NAR was 75 kg/ha, the amount of loss and rate of loss were 26.20 kg/ha and 34.93%; when the NAR was 375 kg/ha, the amount of loss and rate of loss were 168.64 kg/ha and 44.97%. According to the nitrogen loss of different stages of wheat, the nitrogen loss was most from sowing to returning green stage, which increased as the NAR increased, so the basal fertilizer should be limited. The wheat absorbed more nitrogen from jointing to flowering stage, the treatment with lower nitrogen application appeared deficiency, but the nitrogen loss of higher nitrogen application treatment was still severe, there were more inorganic nitrogen in soil. That nitrogen application rate didn't exceed 225 kg/ha was suitable in wheat season.Fifth, nitrogen content in soil profile (NCISP) was mostly in the cultivation soil, with the increase of soil's depth, the NCISP decreased, it reached the minimum at the soil depth of 40~50cm, and then increased slightly. NCISP was lower than before which was under the treatment without nitrogen, it's because of exhaustion effect. Once being applied nitrogen fertilizer, the nitrogen content in the cultivation soil responded to the nitrogen application rate, it increased as the increase of nitrogen application rate, nitrogen of subsoil was also increasing. NCISP under the lower nitrogen treatment which nitrogen application rate was 175 kg/ha and 350 kg/ha in a year increased a little, but NCISP under the higher nitrogen treatment which nitrogen application rate was 700 kg/ha and 875 kg/ha in a year increased significantly. After the fifth harvest of rice, the NCISP under the nitrogen treatment which applied after two years soil exhaustion was lower than both after one year soil exhaustion and continuous nitrogen treatment. In wheat season, the NCISP especially in the surface soil was obvious lower than that in rice season. The effect of soil exhaustion was stronger to wheat yield. Seasonal nitrogen application would change soil nitrogen content, and it was related to nitrogen application rate.Synthesizing all the results of this research, in the condition of higher yield, giving attention to the nitrogen use efficiency, ammonia volatilization losses, the leaching of NO3--N in the soil, the change of inorganic nitrogen in soil profile in wheat season, it was indicated that in Taihu Lake region, if you wanted to achieve the aim of 9750 kg/ha for rice yield and 6000 kg/ha for wheat yield at present, the appropriate nitrogen application rate for rice was 225~300 kg/ha, and 180~225 kg/ha for wheat. With the appropriate nitrogen application rate, the content of background nitrogen in soil would keep steady, crops would get higher yield, nitrogen would get comparatively higher use efficiency and environment would be in a secure state.
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