| Nitrogen is indispensable to the agricultural production. The use of nitrogen improved crop yields, solved the problem of food demands. However, a large number of unreasonable N fertilization not only reduced the N utilization coefficient, but also caused negative influence to surrounding environment, especially the water pollution. Due to the increase of farmland nitrogen rates, the N content in the worldwide surface water and groundwater is in different ascendant trend, and some pollution has reached dangerous level. One of the key technologies to solve these problems is to consider the production, ecological and economic benefits, and to consider the scientific and reasonable irrigation and fertilization, so as to increase production, save water resources and protect the environment quality. Scholars have already conducted extensive research in the N migration and transformation, and achieved successful results. However, the leaching researches under the irrigation and fertilization circumstances in loess soil are not too much. So, this experiment is under the certain nitrogen level and different irrigation level, using indoor soil infiltration simulation experiment to study the N leaching intensity, and to determine the relationship of irrigation, fertilization and N leaching rate, and to provide scientific theory for reasonable irrigation, high water utilization coefficient, and low groundwater pollution.The main conclusions of this article studies are as follows:(1) In different irrigation conditions, the soil moisture in the profile is increasing with the increase of irrigation quota and soil depth. It reflects the process of soil moisture penetration migration in downward. The N content is mainly concentrated below 33cm soil. It shows the traditional irrigation manner is adverse to the shallow soil preservation and crop utilization. It causes soil moisture leakage, reduces fertilizer utilization rate, and causes groundwater pollution.(2) The change of soil profile moisture is big. The soil profile moisture is reducing with time increase in small irrigation conditions. With the increase of irrigation quota, the soil profile moisture is increasing with time increase. The soil profile N content is increasing with the soil depth deepening. In each irrigation conditions, the soil profile N content is reducing in upper soil, and increasing in lower soil with time increase. (3) The influence on irrigation quota to shallow soil N content is big. Soil moisture is essential to N leaching. The leaching quantity is affected by irrigation quota obviously. With the increase of irrigation quota, the soil moisture and soil N leaching quantity is increasing. Therefore, the soil moisture is the important factor to soil N and N loss in farmland ecosystems. In arid and semi-arid regions, the soil moisture is migrating to deep soil with water due to irrigation water infiltration. If the irrigation quota is too high, the groundwater is serious polluted.(4) The soil profile N leaching rate is increasing with irrigation quota increase. When the irrigation quota is 191.1mm and 229.3mm, the N leaching rate is decreasing with the soil depth deepen. There is no extra water in the soil column bottom, soil N don't leaching. When irrigation quota increased to 254.8mm, soil profile N leaching rate exceed 43.17%, there is much supersaturated water outflow in the soil column bottom, and it causes serious leaching phenomenon. Accordingly, when the fertilized N level is 6mg/cm2, the irrigation quota couldn't exceed 229.3 mm.(5) There is a positive correlation between soil N leaching rate and irrigation quota in representative depth. The N leaching rate is big in surface soil, and changes very little. With the deepening of the soil, the soil N leaching rate is increasing with irrigation quota increase, and it forms a linear relation. There is a negative correlation between soil N leaching rate and soil N content in representative depth. With the increase of irrigation quota, the N concentration is decreasing, but the N leaching rate is increasing. The N leaching rate is big in surface soil, and changes very little. With the deepening of the soil, the soil N leaching rate is changing obviously with N concentration decrease, and it forms a linear relation.(6) The loess N leaching rate influencing factors are irrigation, N concentration, soil depth, infiltration time etc. Simulate model in single factor and many factors on the nitrogen leaching through DPS, there is a quaternary quadratic multinomial simulation model in many factors on the nitrogen leaching. By the fitting relationship, there is a positive correlation among soil N leaching rate and irrigation quota, fertilized N concentration, infiltration time. There is a negative correlation between soil N leaching rate and soil depth.
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