A study of tile drain nitrate - δ¹⁵N values as a tool for assessing nitrate sources in an agricultural region

Abstract

It is important to evaluate tools which provide insight into nitrate (NO⁻ ₃) contamination source identification in watersheds where multiple nitrogen (N) sources are applied. As nitrate-N stable isotopes have been previously used to identify contaminant sources in groundwater environments, the application of the technique to tile drainage outflow was investigated. Nitrate-N isotopic and concentration analyses of tile drain discharges from six different fields with a range of mineral fertilizer N and hog manure applications were conducted to examine general isotopic patterns and their relation to N fertilizer sources. δ¹⁵N of NO⁻ ₃ draining fields were compared to δ¹⁵N source signatures through a single growing season. The objective was to determine: (a) whether tile drainage water exiting fields receiving different N sources (inorganic mineral N, organic hog manure N, or a combination of the two) had distinct δ¹⁵N values, and (b) whether δ¹⁵N signatures of sampled tile drain water fell within expected source ranges. Results suggest that isotopic data differed between fields in a manner consistent with differences in NO-3 sources, as fields only fertilized with mineral N had δ¹⁵N values consistently lower than fields with hog manure applications. However, all fields showed isotopic values that were enriched in ¹⁵N relative to their sources during the study period. Therefore, although these fields are discharging tile drainage water with distinctive isotopic signatures, the data suggests that a quantitative evaluation of individual NO⁻ ₃ source contributions is not possible within this watershed. Utilization of this tool in source discrimination in other tile drainage waters should only proceed if it can be demonstrated that isotopic fractionations are not altering source signatures.