The nitrate nitrogen (NO^－_３-N) is one of the main contaminants of ground water in areas of intensive agriculture around the world. And the dynamics and residue of soil NO^－_３-N are related closely to NO－3 leaching especially in intensive cultivated saline soils, where excessive water is usually irrigated in order to prevent salts accumulation in the root zone. To promote the reasonable using of nitrogen (N) fertilizer and agricultural sustainable development in saline soil, it is necessary to study the effects of N fertilizer application rates on soil NO^－_３-N dynamics and residue in this area. So this field trial was conducted in saline fluvo-aquic soil in Nanpi County, Hebei Province, China between June 13-September 28, 2005. Plant used was maize (Zea mays L.). N fertilizer was applied at rates of 0, 70, 140, and 210 kg N hm^－２ as urea on July 14. There were 4 replicate plots of each treatment. For each plot, a soil sample was taken at each layer of soil, with total 6 layers from 0100 cm. There were 9 sampling times throughout the growing season. When irrigating, eight water samples were collected every 10 minutes. Rainwater was also collected with 8 samples each rain time during the experimental period. All of the samples were stored under frozen condition. The NO^－_３-N concentration in the soil and the ammonia and NO^－_３-N concentrations in water were analyzed using Flow Solution IV Analyzer (FSIV, O.I. Analytical, U.S.A.).
The results showed that the trend of the dynamic changes of total amount of NO^－_３-N in 0100 cm soil profile along maize growth proceeding is a wave curve. And applying N fertilizer augmented the wavy extents. N fertilization significantly increased the total amount of NO^－_３-N in soil of each layers from 0100 cm. There was a positive correlation between rates of N application and soil NO^－_３-N concentrations (R=0.834**). The concentrations of NO^－_３-N in the topsoil (040cm) were relatively high within a short period after N application and decreased overtime because NO^－_３-N moved to the deep soil (40100cm). Concentrations of NO^－_３-N in the lower layers of soil were higher at harvest only when N applied. High rates of N application (>70 kg N hm^－２) not only decreased grain yield of maize, but also significantly increased concentrations of NO^－_３-N in soil and resulted in the large amounts of N losing. The study demonstrated that applications of N significantly affected dynamics and residue of NO^－_３-N in this saline soil.