NO3− circulation and associated driving factors in the unsaturated zone of southwestern Tengger Desert, Northwestern China

Abstract

In the southwestern Tengger desert in northwestern China, nitrate circulation processes in the unsaturated zone and associated driving factors were studied using water chemistry and stable isotope techniques. At unvegetated sand sites, NO3− content increases from the south to north in the desert. Additionally, a negative correlation (R2 = 0.90, P < 0.005) was evident between the stability of soil NO3− and elevation, suggesting that NO3− storage in soil is significantly influenced by precipitation and temperature. And a significant positive correlation between mNO2−/Cl− in unsaturated zone and elevation (R2 = 0.50, P < 0.0001), indicating both nitrification and denitrification are strongly in the south of desert, however, the nitrification plays a dominant role in the north of desert. Isotopic analyses (δ15N-NO3−, δ18O-NO3−, and δ18O-H2O, δ2H-H2O) of soil pore-water showed that the two dominant NO3− sources were the mixing of atmospheric deposition and soil nitrification, and that the NO3− circulation at the vegetated sand sites was affected by vegetation and animals. Additionally, local environmental conditions lead to complex driving factors, such as the abiotic process of ammonia volatilization of NO3− circulation, which resulted in the 15N enrichment and 18O depletion of residual nitrate in the unsaturated zone of one of the soil profiles. Denitrification was noted in the deep unsaturated zone of two profiles, indicating that the N loss resulted from abiotic versus biotic processes. Analyses of the stable isotope results and the distribution characteristics of NO3− and Cl− suggests leaching of soil NO3− as a result of NO3− in unsaturated zone poses a potential threat to groundwater quality in Dengmaying region. This study suggests that NO3− circulation in the unsaturated zone responds to climate characteristics in the desert ecosystem, and to provide useful information for the protection of groundwater quality in arid desert regions.