Abstract
Identification of the nitrate sources that adversely impact groundwater quality is a necessary first step in the control of this major worldwide pollutant. The impact of nitrate leachate from urea-ammonium nitrate (UAN) (50% urea-N, 25% ammonium-N, 25% nitrate-N) fertilizer, whose use has increased dramatically in the last three decades largely because it can be applied through sprinkler irrigation systems to corn in all growth stages, is investigated. The dual isotopes δ15NNO3 and δ18ONO3 were measured in groundwater samples from 39 irrigation wells in two intensively sprinkler-irrigated, corn-growing areas of Nebraska with nitrate-contaminated (N > 10 mg/L) groundwater and documented UAN use to ascertain whether nitrified ammonia and nitrate fertilizers can be distinguished in the High Plains aquifer. The areas, which are highly vulnerable to nitrate leaching and differ only in the composition and thickness of their unsaturated zones, are uniquely suited to provide scientific evidence of the feasibility of identifying nitrate fertilizer leachate in groundwater and thereby add significantly to the small body of existing and inconclusive data. The dual isotope method (DIM) results indicate that the nitrate contamination in 38 wells is mostly nitrified ammonium fertilizer. Most importantly, nitrate fertilizer from UAN was not identified isotopically in groundwater beneath almost all fields with documented heavy UAN use. This could be a potentially valuable finding for fertilizer management or it could convey limitations on the appropriateness of the DIM for nitrate fertilizer source identification in groundwater. Slightly enriched δ15NNO3 values in a few wells coincide with the practice of wintering cattle on corn stubble, which reportedly occurred more frequently in one focus area. The absence of natural soil-N leachates and denitrification in groundwater enabled an apparently reliable identification of manure leachates in both areas.
Highlights
Leachate from nitrogen (N)-fertilized, irrigated farmland in Kansas, Nebraska, Oklahoma and Texas is the major nonpoint-source of contamination in the underlying High Plains aquifer (Gurdak and Qi, 2006)
Average δ18OH2O values of −9.5‰ and − 8.5‰ in the groundwater of the Upper Elkhorn Natural Resources District (UENRD) and the Tri-Basin Natural Resources District (TBNRD) focus areas (Tables 1, 2), respectively, are in the middle of the range for mean-weighted precipitation values reported by Harvey and Welker (2000) for northcentral and south-central Nebraska, respectively
(1987) formula, average δ18Oatm values in the UENRD and TBNRD focus areas ranged from +19 to +21‰ and are within the +18 to +22‰ range reported by Amberger and Schmidt (1987)
Summary
Leachate from nitrogen (N)-fertilized, irrigated farmland in Kansas, Nebraska, Oklahoma and Texas is the major nonpoint-source of contamination in the underlying High Plains aquifer (Gurdak and Qi, 2006). Since 1980 urea-ammonium nitrate fertilizer (UAN) (50% ureaN, 25% ammonium-N, 25% nitrate-N) has increasingly replaced anhydrous ammonia as irrigated-corn producers' commercial N fertilizer of choice, and by 2005 it was the predominant commercial N fertilizer applied in Nebraska (Ferguson, 2015). Δ15N values for fertilizers sold in Nebraska range from −2 to +2‰ for urea-N and ammonium-N in UAN and from +3 to +7‰ for nitrate fertilizer (Spalding et al, 1982). Worldwide nitrate fertilizer δ15N values average + 2.75‰ (Hübner, 1986). Nitrate is the most-enriched commercial N fertilizer formulation reported (Mariotti and Letolle, 1977; Spalding et al, 1982; Hübner, 1986; Fernández et al, 2017). The nitrate and ammonium components of UAN, the only nitrate fertilizer
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
