Migration and transformation of nitrate at the typical geomorphic types of deep vadose zone in the plain area of Baiyangdian Watershed

Excess nitrogen is accumulated in the vadose zone resulting in the gradual increase of NO₃⁻-N accumulation due to the overuse of nitrogen fertilizer. The Baiyangdian Basin is located in the western part of the North China Plain, has been impacted by over-exploitation groundwater resulting in decrease of groundwater levels and increase of vadose zone thickness. However, at a large spatial scale, the understanding of mechanism and controlling factors of nitrogen leaching and transformation affected by the difference of geomorphic types and soil texture is still unclear until now. In this study, we selected 9 deep soil profiles (20 m depth) through two typical alluvial-proluvial fan systems (including the loessal terrace, alluvial-proluvial fan, flood plain, and lake depressions) in the Baiyangdian Lake Watershed to investigate the accumulation, leaching, and transformation processes of NO₃⁻-N by the hydrochemical analysis and the chloride ion balance method. Results shows: 1) Soil texture of soil profile, influenced by geomorphic types, has a spatial difference with sand-texture of alluvial-proluvial fan and clay-texture of flood plain to determining the spatial distribution of NO₃⁻-N accumulation and leaching in the vadose zone. The NO₃⁻-N accumulation ranges from 352.7 to 3276.7 kg·hm⁻² in the soil profiles of 20 m depth, with the maximum accumulation occurring in the flood plain and lake depressions. Conversely, NO₃⁻-N leaching rate below the root zone ranges from 9.8 to 252.0 kg·hm⁻²·a⁻¹, with the maximum leaching occurring in alluvial-proluvial fan. The NO₃⁻-N accumulation shows increasing trend while NO₃^--N leaching shows decreasing trend from upstream to downstream in soil profiles controlled by the same geomorphic types. The nitrogen accumulation of orchards (1544.0~3133.8 kg·hm⁻²) is higher than that of vegetables (2641.6 kg·hm⁻²) and wheat-maize (352.7~3276.7 kg·hm⁻²) among different land use types. 2) The transformation of nitrogen in the deep vadose zone of Baiyangdian Lake Watershed is mainly influenced by crop absorption, nitrification and denitrification. Nitrification occurs in the whole soil profiles of 20 m depth, particularly in alluvial-proluvial fan with sand-texture layers. The variation of peak value in deep sandy loam soil of alluvial-proluvial fan area is caused by the joint effect of nitrification and denitrification. However, the denitrification occurred mainly in the vadose zone above 10 m depth in flood plain or lake depressions of clay-texture layers due to the decreasing of soil organic carbon from the surface to the deep vadose zone. This research not only advances current research about nitrate migration and transformation from point-scale to watershed scale but also provides valuable insights for future investigations in the physical and geochemical processes of nitrogen leaching at a watershed scale. Additionally, this study also provides scientific support for the control of agricultural non-point source pollution, protection of water environment, and integration of land use management in the Baiyangdian Watershed.