NO 3 − sources and circulation in the shallow vadose zone in the edge of Dunhuang Mingsha sand dunes in an extremely arid area of Northwestern China

Abstract The unsaturated zone is a significant feature of the nitrogen cycle in arid areas connecting atmospheric precipitation and surface water to groundwater. To measure NO 3 − , Cl − , and NH 4 + concentrations, precipitation samples (39.9592°N, 94.3302°E) were collected at the western edge of the Mingsha sand dunes. Soil profiles were taken along the northern edge of the dunes to determine NO 3 − and Cl − concentrations and δ 18 O-NO 3 − and δ 15 N-NO 3 − levels in soil water. The mNO 3 − /Cl − , mNH 4 + /Cl − and NH 4 + -N/NO 3 − -N values in precipitation showed seasonal variations, reflecting the influence of distinct anthropogenic sources. The δ 15 N-NO 3 − and δ 18 O-NO 3 − of the LT profile ranged from 2.38 to 7.84‰ and 14.72 to 57.5‰, respectively, and that of GC profile ranged from − 1.36 to 2.35‰ and 5.07 to 21.3‰, respectively. These results suggest that the main sources of vadose zone nitrogen in the profiles are NO 3 − fertilizer and NH 4 + in fertilizer and precipitation, all of which are influenced by agricultural sources. Obvious nitrification and the mixing of different sources were noted, but the profiles showed no evidence of denitrification. The findings of this study show that migration of NO 3 − and Cl − in the desert vadose zone is influenced by heavy rainfall events, vegetation, and evaporation. Heavy rainfall events promote leaching of NO 3 − and Cl − into the deep vadose zone, while evaporation at the surface prevents leaching and vegetation hinders the downward migration of NO 3 − and Cl − . Migration of NO 3 − -N and Cl − in sand dunes is affected by the solute gradient concentration difference of the soil water, moving from areas of higher concentrations to lower concentration. The NO 3 − -N and Cl − accumulations were 1.02 kg N ha − 1 and 3.45 kg ha − 1 , respectively, throughout autumn. These were larger than could be attributed to the input flux of atmospheric precipitation, especially in the near-surface soil layer, likely illustrating the influence of atmospheric dry deposition.