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

The Baiyangdian Lake watershed is located in the western of the North China Plain, has been impacted by the over-exploitation of groundwater and over-use of agricultural N-fertilization resulting in decrease of groundwater levels and increase of vadose zone thickness. However, the leaching processes and transformation mechanisms of nitrate in chick vadose zone which was controlled by geomorphic sediments from the viewpoint of watershed is still unclear. This study investigated the accumulation, leaching, transformation processes, and influencing factors of nitrate-nitrogen (NO₃-N) in 9 deep soil profiles (0-20 m) across two typical alluvial-proluvial fan system with different geomorphic types (alluvial-proluvial fan, flood plain, lake depressions) in the plains of the Baiyangdian Basin by using the hydrochemical analysis and chloride balance methods. Results show: 1) Soil texture, influenced by geomorphic types, affects 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·ha^-1 in the soil profiles with 20 m depth, and NO₃-N leaching rate below the root zone ranges from 9.8 to 252.0 kg·ha^-1·y^-1. The NO₃-N accumulation shows increasing trend while NO₃-N leaching shows decreasing trend from upstream to downstream of the alluvial-proluvial fan system; 2) Nitrification occurs throughout the soil profiles of 20 m depth, particularly in alluvial-proluvial fan with sand-texture layers where nitrification causes variations of NO₃-N peak. However, denitrification, limited by lack of soil organic carbon, mainly occurred in the vadose zone above 10 m depth in flood plain of clay-texture layers. This research provides scientific support for control of non-point source pollution and protection of water environment in the Baiyangdian Basin and the Xiong’an New Area.