Abstract: 【Objective】The aim of this article was to determine clear characteristics of input and leaching of water and nitrogen under a typical farmland planting system in the North China Plain, and clarify the effect of N fertilizer on leaching water quality, which was eventually to provide a scientific basis for reducing nitrogen leaching from the source and prevention farmland of non-point source pollution. 【Method】 The experiment was set in the “Key Experimental Station on Ecological Environment of Drab Fluvo-aquic Soil in Changping, Ministry of Agriculture” for maize monoculture from 2007 to 2012, including three nitrogen gradients (no nitrogen CK, optimize nitrogen T1 and traditional nitrogen T2). A large lysimeter was used to monitor the process of leaching of soil water and nitrogen during maize growth, at the same time the field microclimate stations were used to monitor the process of precipitation, while detailed irrigation events were also recorded. 【Result】The result showed that the precipitation and irrigation mainly occurred in April to October of every year. And leaching mainly occurred in May to November, which was later than the precipitation and irrigation. Annual precipitation was different among the five years (134-525 mm). Precipitation in 2009 was less with no irrigation and resulted in no leaching events, while total water input was higher than 500 mm/year in other years leading to leaching events. Leaching occurrence frequency and water amount in the treatment of nitrogen application decreased compared to no nitrogen application, CK had 37 leaching events with a water volume of 422 mm while T2 had 31 leaching events with a water volume of 310 mm. Total nitrogen in irrigation and precipitation was 157 kg·hm^-2 and the dissolved total nitrogen was 106 kg·hm^-2, while annual total nitrogen in irrigation and precipitation was different among the five years (7.53-34.1 kg·hm^-2). For one year, the more nitrogen in irrigation and precipitation, the more nitrogen leaching for the same treatment. Nitrate-N in dissolved total nitrogen was the main type of nitrogen, the ratio of nitrate-N in the leaching water increased with the increase of the nitrogen application. No matter in single or annual leaching events both dissolved total nitrogen and nitrate-N in leaching water of T2 was significantly higher than that of CK and T1. Average nitrate-N concentration of CK and T1 in all leaching events were lower than 2.0, 5.0 mg·L^-1 respectively, while the average nitrate-N concentration of 31 leaching events of T2 within five years was 29.5 mg·L^-1. There were 15 leaching events in which the nitrate-N concentration was higher than Class III of groundwater (20 mg·L^-1), of which the maximum concentration was 79.0 mg·L^-1. 【Conclusion】 Irrigation and precipitation was the main cause of leaching, and the more water input, the more leaching. Nitrate-N was the main type of nitrogen in leaching water. In different years, nitrogen leaching had a positive relationship with dissolved total nitrogen in precipitation and irrigation. The more dissolved total nitrogen involved in precipitation and irrigation, the more nitrogen leaching. In one year, the amount and concentration of nitrate-N leaching increased with the increase of the nitrogen application rate. So nitrogen application rate must be reduced to decrease nitrogen leaching and ensure the water nitrate content does not exceed the criteria. However, the effect of the total nitrogen involved in precipitation and irrigation on nitrogen leaching when identifying the optimal nitrogen rate should be considered.

Key words: North China Plain, precipitation, nitrogen application, dissolved total nitrogen, nitrate nitrogen, leaching, water quality