Leaching losses of nitrate nitrogen and dissolved organic nitrogen from a yearly two crops system, wheat-maize, under monsoon situations

A large amount of nitrogen (N) fertilizers applied to the winter wheat-summer maize double cropping systems in the North China Plain (NCP) contributes largely to N leaching to the groundwater. A series of field experiments were carried out during October 2004 and September 2007 in a lysimeter field to reveal the temporal changes of N leaching losses below 2-m depth from this land system as well as the effects of N fertilizer application rates on N leaching. Four N rates (0, 180, 260, and 360 kg N ha^-1 as urea) were applied in the study area. Seasonal leachate volumes were 87 and 72 mm in the first and second maize season, respectively, and 13 and 4 mm during the winter wheat and maize season in the third rotational year, respectively. The average seasonal flow-weighted NO₃-N concentrations in leachate for the four N fertilizer application rates ranged from 8.1 to 103.7 mg N l^-1, and seasonal flow-weighted dissolved organic nitrogen (DON) concentrations in leachate varied from 0.8 to 6.0 mg N l^-1. Total amounts of NO₃-N leaching lost throughout the 3 years were in the range of 14.6 to 177.8 kg ha^-1 for the four N application rates, corresponding to N leaching losses in the range of 4.0-7.6% of the fertilizers applied. DON losses throughout the 3 years were 1.4, 2.1, 3.6, and 6.3 kg N ha^-1 for the four corresponding fertilization rates. The application rate of 180 kg N ha^-1 was recommended based on the balance between reducing N leaching and maintaining crop yields. The results indicated that there is a potential risk of N leaching during the winter wheat season, and over-fertilization of chemical N can result in substantial N leaching losses by high-intensity rainfalls in summer.