In-situ nitrogen fate in the vadose zone of different soil types and its implications for groundwater quality in the Huaihe River Basin, China

This paper focused on nitrate fate in the vadose zone (VZ) and its implications for groundwater vulnerability under different soil types in the agricultural area of Huaihe River Basin, China. Isotopic compositions of nitrate (δ 15 N and δ 18 O) along with NO 3 − and Cl − concentrations were determined in the VZ-shallow groundwater continuum beneath silty-loam and silty-clay-loam, which are distinctive in texture and organic carbon (OC). In the soil zone (< 1 m in depth), measured δ 18 O–NO 3 − suggested the ubiquitous of nitrification regardless of soil types. In the subsoil zone (> 1 m in depth), however, the concurrent enrichment of δ 15 N–NO 3 − and δ 18 O–NO 3 − indicated the occurrence of denitrification, which showed a dependence on subsoil properties. Specifically, during wheat and maize land uses, denitrification removed as much as 76 %–88 % of the total nitrate where the subsoil was dominated by stratified OC-rich silty-clay-loam. In contrast, only 0 %–28 % of the nitrate was degraded via denitrification where the subsoil was composed of uniform, OC-depleted silty-loam. Furthermore, inactive denitrification and higher permeability in the silty-loam VZ implied higher groundwater vulnerability. This observation was consistent with the fact that groundwater NO 3 − –N concentration beneath silty-loam (11.24 mg L −1 ) was over two times higher than that of the silty-clay-loam (5.32 mg L −1 ), where stricter fertilization management and conservation strategies should be applied to protect groundwater quality.