Abstract:In the high yield farmland ecosystem，the dynamics of nitrate nitrogen distribution in soil profile influenced by nitrogen fertilization and crop straw returning practice is studied by field plot experiment．At a certain nitrogen rate，the content of NO ₃ ^--N decreases with depth，which can be regressively described as a power function Y=aX ^b．Though little difference is made between the crop straw returning patterns of either wheat-corn and wheat alone，more nitrogen leaching tends to occur in the later practice since more nitrate nitrogen have been accumulated within 0～40cm soil layer．Soil content of NO ₃ ^--N reaches the summits twice and the valley once in a year．The summits turn up after harvest stages of corn and wheat and the valley lasts from wheat seedling in spring to flowering，when wheat shows the maximum uptake of available nutrient from soil body．Content of NO ₃ ^--N fluctuates most within 0～40cm layer and almost keeps stable below 60cm depth．The fluctuation of content of NO ₃ ^--N within the observed year through soil profile as deep as two meters is closely correlated with the rate of nitrogen applied．Until the end of September after raining season，content of NO ₃ ^--N is up to lOmg／kg．It is therefore predicted that under the conditions of present farmland ecosystem soil nitrogen is leached not only to two meters depth but also out of two meters soil body．