Study of nitrate leaching potential from agricultural land in Northern China under long-term fertilization conditions

N balance and the potential of nitrate leaching loss(PNLL) under winter wheat and summer maize rotation system in Northern China was studied with two long-term fertilization experiments,one is at fluvo-aquic soil in Xinji County of Hebei Province with 21 years and another is at brown fluvo-aquic soil in Changping District of Beijing City with 9 years.It was shown that crop yield was lowest under CK treatment and somewhat can be increased by P or PK fertilization.Single N application has little positive effect on crop yield which even brought on negative effect in Xinji experiment.As a result,N surplus was as high as 137 and 212 kg/ha per year under single N fertilization for Xinji and Changping respectively.Crop yield was remarkably increased under NP or NPK fertilization,but N surplus still remain at a lower level even though N uptake improved.When suitable amount of manure was applied combined with NP or NPK fertilizer,crop yield reach very high,but excessive manure application almost has no influence on crop yield,but N surplus can be as high as 227 and 231 kg/ha per year for the two experiments,even much higher than that of single N fertilization.The PNLL was most serious when only N fertilizer was applied in the two sites,as shown that the peak of nitrate content(PNC) occurred at 160200cm and 120160cm in soil profile and most of that was in the deep soil layer.When P or PK was applied with N fertilizer,the PNC went upwards for 3040cm,also nitrate cumulation was sharply decreased both inside and outside the root zone,thus the PNLL was lowered down remarkably.When suitable amount of manure was applied with NP or NPK fertilizer,the NPC went up to 090cm,nitrate cumulation outside the root zone was continuously lowered down,the PNLL was very weak.But when excessive manure or N fertilizer was applied,much more nitrate was accumulated in the deep soil layers and the PNLL became much stronger.Heavy soil texture could lower the PNLL.It can be inferred that balanced fertilization is not only an effective way for increasing crop yield,but also a practical measure to control nitrate leaching loss in field scale.