Geomorphologic and sedimentary features dominate the nitrogen accumulation and leaching in the deep vadose zone from a catchment viewpoint

Abstract

Although the application of nitrogen fertilizer increases grain yield, it also raises the risk of nitrogen leaching to groundwater. Not much work has been done on nitrogen accumulation and leaching at a watershed level, especially for deep vadose zone of alluvial-proluvial sediments. Here, nitrate (NO₃^–) accumulation and leaching were investigated in the deep vadose zone (20 m below ground level (b.g.l.)) across Baiyangdian watershed in the North China Plain (NCP). The study area is rich in sedimentary deposits of varying geomorphologic features. Results of the study showed that watershed geomorphologic and sedimentary features control not only soil water flux, but also nitrate accumulation and leaching in the deep vadose zone. Nitrate leaching was highest (252.0 kg·N·ha⁻¹·y⁻¹) and accumulation lowest (352.7 kg·N·ha⁻¹) in regions of the study area with highly permeable sandy alluvial-proluvial fan. In contrast, nitrate accumulation was highest (3276.7 kg·N·ha⁻¹) and leaching lowest (9.8 kg·N·ha⁻¹·y⁻¹) in thick silty/clay flood plains and lake depressions areas. The nitrate flux was primarily driven by vertical infiltration in flood plains and lake depressions. Also, Nitrogen fertilizer (N-fertilizer) input and irrigation affect nitrogen leaching into the deeper vadose zone and groundwater in the study area. It was inferred that, rising of water table under extreme precipitation events could trigger the release accumulated soil nitrogen, the shorten nitrogen leaching lag-time, and increase risk of groundwater pollution. This finding could guide policy decisions given the sensitivity and vulnerability of groundwater to the risk of pollution in the study area.