A study was conducted in Beijing, China to determine the nitrate content in 254 soil profiles under different land use types, using global positioning systems and deep soil core sampling, for the forecasting of the potential of nitrate pollution in groundwater . NO 3--N accumulation in 0-400 cm soil profiles in protected vegetable fields was the highest (1230 kg/ha) while it was lowest (69 kg/ha) in paddy fields. Even for the same land use types, NO 3- accumulation among sampling fields differed significantly. The percentage of soil profiles with NO 3--N accumulation more than 800 kg/ha was 70.4, 50.0 and 33.3 in protected vegetable field, orchards and open vegetable fields, respectively, while it was only 8.6 and 125 in winter wheat-summer maize rotation fields and spring maize fields, respectively. NO 3--N content reduced sharply with the depth of the soil profile in dryland fields. Although there was no remarkable difference in the ratio of NO 3--N content in 200-400 cm soil layer to the whole profile among various land use types, NO 3--N accumulation rates differed distinctly. The highest was protected vegetable fields and orchards while the lowest was observed in winter wheat-summer maize rotation fields and spring maize fields. The results indicate that protected vegetable fields pose a serious threat to groundwater pollution while cereal fields have lesser risks.
