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A 1.6 ha plot of A triplex canescens (fourwing saltbush) was established in a desert soil at a former uranium ore-processing plant, near Monument Valley, Arizona, to remediate nitrate and ammonium N contamination. The plants were irrigated to stimulate growth and N uptake. However, NO3- loss from the soil was unexpectedly rapid. Initially, the soil contained approximately 180 mg kg(-1) NO3--N distributed at depths up to 4.6m, but concentrations decreased to 80 mg kg(-1) after 41 months. Losses occurred throughout the plot at all soil depths. NH4-N remained unchanged (ca. 180 mg kg(-1)). Soil moisture was generally below field capacity and soil water flux showed no net downward movement over the course of the study. A salt balance showed a 10% decrease in soluble salts during the study, attributable to the loss in NO3-. Residual soluble soil N became progressively enriched in N-15 over time, consistent with biological denitrification. Additionally, microcosm studies indicate significant potential denitrification rates on the plot but not for control soils. Total losses of NO3--N were 1360 kg ha(-1) yr(-1). These findings of rapid denitrification in the vadose zone of a desert soil are unique and may offer a low-cost method for NO3- remediation at similar sites. These findings are also of interest due to the depth at which the losses occurred and the possibility that considerable amounts of naturally occurring NO3- in deep vadose zone desert soils in the southwestern US could be similarly mobilized by changing land use practices or climate change. (C) 2004 Elsevier Ltd. All rights reserved.