Distinguishing and quantifying the fate of nitrate in irrigation water and nitrate produced by ammonium nitrification

When the sources of nitrogen include not only ammonium (NH₄⁺) fertilizer (ANF) but also nitrate (NO₃^-) from groundwater and rainfall (NRI), if the proportions of various types of NO₃^- are still based on the amount of ANF, the corresponding calculation method may be complicated. This paper established a water flow-nitrogen migration transformation model for the unsaturated zone in grain-planting and vegetable-planting areas, and studied the migration and transformation of NH₄⁺ and NO₃^- in the unsaturated zone when ANF and NRI coexist. This paper proposed for the first time the proportional coefficient method (PCM) and hypothetical assignment method (HAM) to distinguish and quantify the fate proportions of NO₃^- from NO₃^- produced by NH₄⁺ nitrification (NNR) and NRI. The results showed that the PCM was more practical than the HAM in quantifying the fate of NO₃^- from different sources. If only the root absorption ratio was used to evaluate the degree of nutrient supply to crops, the ratios of root absorption were as high as 40% (44.75-50.85%). NRI provided more nutrients in grain-growing areas than those in vegetable-growing areas. If the sum of the proportion of other fates was regarded as the degree of groundwater NO₃^- mitigation through irrigation in the unsaturated zone, except for the ratio of NO₃^- leaching to groundwater, the proportion of NO₃^- pollution mitigation was as high as 57.89% (57.89-92.99%), and the mitigation ability of groundwater NO₃^- pollution in grain-growing areas was higher than that in vegetable-growing areas.