A field experiment was conducted in the North China Plain to investigate effect of N application rate on soil NO₃^--N and its distribution in the soil profile under a maize-winter-wheat cropping system.The potential hazard of soil NO₃^--N to the environment was also evaluated.Soil NO₃^--N contents were measured of soil layers,20 cm each,of a 180-cm deep soil profile.After the crops were harvested soil NO₃^--N residues in the profile varied greatly from year to year and from crop to crop in the field experiment with values ranging from 107 to 443 kg/hm².A general trend of soil NO₃^--N content increasing with N application rate was observed,while significant difference was hardly observed.During the experiment,a peak soil NO₃^--N content of 688 and 881 kg/hm² for present local farmers' traditional N rate and 2 times the traditional N rate, respectively,was observed in the growth period of maize in 2003.It is generally considered that the soil NO₃^--N within the layer of 100~180 cm could easily transport by leaching to the environment that beyond the bottom of the plant-soil system or be regarded as potential N source for subsequent crop growth.About 50% of the soil NO₃^--N content at layers of 0~180 cm was accumulated in the deeper layers of 100~180 cm during the growth period of maize in 2003,mainly caused by the heavy rainfall occurred concurrently.It was reported that such kind of heavy rainfall occurred with high frequency.Therefore,potential contamination with NO₃^--N might occur even with local farmers' traditional management strategy. Further research is required to investigate the contamination degree.