Abstract:

Aerobic conditions in desert aquifers commonly allow high nitrate (NO₃⁻) concentrations in recharge to persist for long periods of time, an important consideration for N-cycling and water quality. In this study, stable isotopes of NO₃⁻ (δ¹⁵N_NO3 and δ¹⁸O_NO3) were used to trace NO₃⁻ cycling processes which affect concentrations in groundwater and unsaturated zone moisture in the arid Badain Jaran Desert in northwestern China. Most groundwater NO₃⁻ appears to be depleted relative to Cl⁻ in rainfall concentrated by evapotranspiration, indicating net N losses. Unsaturated zone NO₃⁻ is generally higher than groundwater NO₃⁻ in terms of both concentration (up to 15 476 µM, corresponding to 3.6 mg NO₃⁻-N per kg sediment) and ratios with Cl⁻. Isotopic data indicate that the NO₃⁻ derives primarily from nitrification, with a minor direct contribution of atmospheric NO₃⁻ inferred for some samples, particularly in the unsaturated zone. Localized denitrification in the saturated zone is suggested by isotopic and geochemical indicators in some areas. Anthropogenic inputs appear to be minimal, and variability is attributed to environmental factors. In comparison to other arid regions, the sparseness of vegetation in the study area appears to play an important role in moderating unsaturated zone NO₃⁻ accumulation by allowing solute flushing and deterring extensive N₂ fixation.