U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting, Colorado Springs, Colorado, September 20-24, 1993, Water-Resources Investigations Report 94-4015

Use of 15N to Trace Movement of Nitrogen Fertilizer at a Field Plot

Abstract

A simplified, lower-cost technique forpreparation of water samples for nitrogen isotope (¹⁵N) analysis of nitrate (NO₃) is being tested. Anion exchange resins were used to concentrate nitrate in the field and samples were prepared for ¹⁵N - NO₃ (ratio of ¹⁵N to ¹⁴N in NO₃ , relative to atmospheric N, expressed in per mil) analysis using a modification of the Dumas combustion method. Initial test results of the ¹⁵N - NO₃ technique indicate that a precision of 0.05 per mil can be achieved. More than 99 percent of nitrate in solution is sorbed onto the resins. Hydrochloric acid was used to elute 95 to 100 percent of the nitrate from the resins. Chloride and organic molecules can interfere with ¹⁵N - NO₃ values, but not at concentrations detected at the field plot.

The ¹⁵N-labeled fertilizer was applied to a 0.1-acre field plot in May 1992 to determine the quantity of nitrogen fertilizer reaching ground water. Twenty-two percent of the labeled fertilizer was removed from the plot in harvested grain. Soil-gas analyses indicated no significant denitrification of the applied fertilizer. Labeled fertilizer accounted for 34 percent of the nitrate in runoff in the first storm after application. Small quantities of labeled fertilizer were detected 1.5 feet below ground surface 1 month after application, but evapotranspiration prevented further downward movement until November 1992. Labeled fertilizer was detected in shallow wells within 6 days of the first recharge event in November 1992. The ¹⁵N - NO₃ values in deep wells increased 1 month later. Labeled fertilizer accounted for as much as 0.74 milligram per liter of the nitrate in ground water.