An Evaluation of Sources of Nitrogen in Shallow Groundwater Using ¹⁵N Abundance Technique

Abstract

A ¹⁵N abundance technique was employed to identify the source of NO₃-N in groundwater under three commercial citrus production sites in central Florida. Water samples were collected from 0 to 300 and 300 to 600 cm depths in the surficial aquifer and analyzed for NO₃-N and delta N-15 (δ ¹⁵N). Groundwater samples were also collected in a residential area adjacent to one of the citrus groves and analyzed for NO₃-N and δ ¹⁵N. The δ ¹⁵N values were in the range of +1 to +10% in both depths underneath the citrus groves. The range of δ ¹⁵N measured in this study represents the range expected for groundwater that was impacted by NO₃-N originated from mineralization of organic N from the soil as well as from the crop residue. There are occasional high δ ¹⁵N values which are indicative of the effects of NH₃ volatilization losses of applied fertilizer N. The range of δ ¹⁵N values for groundwater samples collected from the residential area adjacent to the citrus groves was very similar to that from the groundwater underneath the citrus groves. Thus, the source of NO₃-N that impacted the groundwater under the citrus groves also impacted the groundwater in the adjacent residential area.

Key Words :

• Nitrate contamination of groundwater
• Nitrogen best management practice
• Nitrogen mineralization
• Non-point source pollution

ACKNOWLEDGMENTS

This study was supported, in part, by a grant from the Florida Department of Environmental Protection (FL DEP). We appreciate the assistance of Bill Potiar for ¹⁵N analysis. The installation of groundwater sampling wells, and nutrient and irrigation management practices are funded by the Florida Department of Agriculture and Consumer Services (FL DACS) as a part of a parallel ongoing project. We extend our appreciation to R. Budell, M. Czerniak, W. Riotte, M. Thomas, T. Hurner, and M. Still. Appreciation is also extended to the citrus growers in central Florida for their cooperation by making their groves accessible to us for this study.

Notes

*Represents standard error of the mean (n = 12; four samples per well × three samplings).