Evaluating nitrate transport and accumulation in the deep vadose zone of the intensive agricultural region, North China Plain

Abstract

Evaluation of the nitrate transport process in the deep vadose zone (DVZ) is important for groundwater quality management, especially in intensive agricultural regions, such as the North China Plain (NCP). The NCP produces ~20% of the total food grain in China, owing to timely groundwater irrigation and excessive chemical N fertilizer applications, and faces severe groundwater environmental degradation. This study evaluated the potential impacts of intensive agriculture on groundwater quality by investigating nitrate accumulation and transport in the DVZ of wheat-maize double-cropping field based on sediment sampling (maximum depth of 45.2 m) over three sub-regions of the NCP. The results showed that legacy nitrate‑nitrogen (NO3−-N) accumulated in the DVZ ranged from 118.5 to 6302.8 kg N ha−1 across the NCP; it increased with depth at an average rate of ~157 kg ha−1 m−1. Nitrate transport and accumulation in the DVZ were spatially varied and mainly controlled by the DVZ sediment textures, in addition to water and nitrogen inputs from the ground surface. Coarse sediments retained lower soil water content, resulting in less nitrogen storage; however, they provided greater nitrate transport velocity. Higher transport velocities observed in the alluvial-proluvial fan allowed chemical N fertilizer to reach the water table. However, in other regions, nitrate transport velocities were lower than the water table decline rates, implying that groundwater quality may not have been impaired by chemical N fertilizer. Furthermore, a reductive environment was identified in some areas with fine sediments, indicating a favorable environment for denitrification in the DVZ. The findings of the current study could provide an important foundation for groundwater quality management in agricultural areas, such as the NCP and similar regions.