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, and 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 has predominantly focused on the piedmont plain. Studies on the central plain, where the groundwater table is relatively shallow and the risk of groundwater nitrate contamination is high, remain limited. This study focused on a typical winter wheat-summer maize rotation field in the central zone of the NCP. Based on field sampling of the thick vadose zone (13 m depth, with three replicates) and laboratory physicochemical analyses, we characterized the nitrogen speciation and accumulation characteristics in the profile. The migration and transformation processes of nitrogen in the deep vadose zone were determined based on the Chloride Mass Balance (CMB) method and environmental factor analysis of the profile. The results demonstrated that the nitrate nitrogen content was the highest in the root zone (0–2 m). In the 2–5.5 m soil layer, nitrate nitrogen content remained stable at a relatively high level. Notably, below 5.5 m, the nitrate nitrogen content decreased rapidly and remained at a lower value. The accumulated amount of nitrate nitrogen above the 5.5 m depth accounts for 86.5% of the total accumulation in the entire profile. Based on the hydrological processes, the sharp decline in nitrate nitrogen concentration below 5.5 m was not due to the failure of heavily applied fertilizers to reach this depth but rather to the nitrogen transformation processes. Furthermore, between 5.5 and 8 m, the oxygen content and redox potential declined significantly, whereas the dissolved organic carbon concentration was relatively high. These conditions were conducive to nitrogen transformation processes under anaerobic conditions, including denitrification, dissimilatory nitrate reduction to ammonium, anaerobic ammonium oxidation, and anaerobic mineralization of organic nitrogen. The enrichment of nitrogen and oxygen isotopes of nitrate at this depth further supported the occurrence of these 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. These findings provide crucial scientific insights for accurately assessing the long-term impacts of agricultural non-point source pollution on groundwater and for developing targeted management strategies.