LI X, LIU M Y, MIN L L, SHEN Y J. Nitrogen transport and transformation processes in the typical deep vadose zone in the central North China Plain[J]. Chinese Journal of Eco-Agriculture, 2026, 33(1): 1−12. DOI: 10.12357/cjea.20250148
Citation: LI X, LIU M Y, MIN L L, SHEN Y J. Nitrogen transport and transformation processes in the typical deep vadose zone in the central North China Plain[J]. Chinese Journal of Eco-Agriculture, 2026, 33(1): 1−12. DOI: 10.12357/cjea.20250148

Nitrogen transport and transformation processes in the typical deep vadose zone in the central North China Plain

  • The North China Plain (NCP) is a key agricultural production area in China, where long-term intensive fertilization and low nitrogen fertilizer use efficiency have resulted in significant nitrogen accumulation in the vadose zone, particularly in the deep vadose zone below the root zone. Nitrogen accumulation poses a serious threat to the regional groundwater environment. Research on nitrogen transport and transformation in the deep vadose zone of the NCP was predominantly focused on the piedmont plain. Studies on the central plain remain limited, where the groundwater table is relatively shallow and the risk of groundwater nitrate contamination is higher. This study focuses on a typical winter wheat-summer maize rotation field in the central zone of the NCP. Based on the field sampling of the thick vadose zone (13-meter depth, with three replicates) and laboratory physicochemical analyses, we characterize the nitrogen speciation and accumulation characteristics in the profile. Based on the chloride mass balance (CMB) method and environmental factor analysis of the profile, the migration and transformation processes of nitrogen in the deep vadose zone were revealed. The results demonstrate that nitrate nitrogen content is highest in the root zone (0−2 m). In the 2−5.5 m layer, nitrate nitrogen content remains stable at a relatively high level. It is worth noting that below the 5.5 m, the nitrate nitrogen content decreases rapidly and maintains at a lower value. The accumulation amount of nitrate nitrogen above the 5.5 m depth accounts for 86.5% of the total accumulation amount of the entire profile. Based on the hydrological processes, the sharp decline in nitrate nitrogen content below 5.5 m was not due to the fact that the high-intensity application of chemical fertilizers did not arrive at this depth, but rather related to the nitrogen transformation processes. Furthermore, between 5.5 and 8 m, oxygen content and redox potential significantly decline, while dissolved organic carbon concentration is relatively high. These conditions are conducive to nitrogen transformation processes under anaerobic conditions, including denitrification, nitrate reduction to ammonium, anaerobic ammonium oxidation, and organic nitrogen anaerobic mineralization. Enrichment of nitrogen and oxygen isotopes of nitrate at this depth can also explain significant nitrogen transformation processes. The nitrogen migration and transformation processes identified in this study determine the fate of nitrogen in the deep vadose zone and its impact on the regional groundwater environment, and may jointly play a barrier role in groundwater quality. The findings provide crucial scientific insights for accurately assessing the long-term impact of agricultural non-point source pollution on groundwater and for developing targeted management strategies.
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