Rapid nitrate loss from a contaminated desert soil

A 1.6 ha plot of Atriplex 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 4.6 m, 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 15N 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.