Deep nitrate accumulation in typical black soil critical zones of Northeast China

Deep nitrate accumulation below 1 m has been observed in various soil regions, yet remains undocumented in the black soil (mainly Phaeozems and Chernozems) region. Climatic and edaphic factors likely influence deep nitrate accumulation on a large scale, although existing studies primarily focus on individual sites. In order to evaluate the distribution and controlling factors of deep nitrate in the black soil region, inorganic nitrogen forms and regolith properties of nine boreholes spanning humid, semi-humid, and semi-arid areas in Fujin, Hailun, and Lindian in northeast China were analyzed down to a depth of 10 m. The results revealed significant nitrate accumulation in Lindian, peaking at 11.03 mg N kg^-1 at a depth of 3 m underground. Nitrate storage from the land surface to a depth of 10 m in Lindian ranged from 459.65 kg N ha^-1 to 1072.88 kg N ha^-1, with over 70 % of nitrate stored below 1 m. Nitrate accounted for 97.74 % of the total N stock in Lindian. Ammonium accumulation has been observed at a deeper depth in Hailun, with no nitrate accumulation detected in Hainlun and Fujin. Regolith properties such as clay, silt, sand, and pH playing a crucial role in reshaping the vertical pattern of nitrate. The presence of nitrate pools at greater depths in intensively managed black soil regions should be taken into account for the sustainable utilization of soil resources and the mitigation of groundwater pollution risks.