Riparian Groundwater Nitrogen (N) Isotopes Reveal Human Imprints of Dams and Road Salt Salinization

Groundwater nitrate‐N isotopes (δ 15 N‐) have been used to infer the effects of natural and anthropogenic change on N cycle processes in the environment. Here we report unexpected changes in groundwater δ 15 N‐ for riparian zones affected by relict milldams and road salt salinization. Contrary to natural, undammed conditions, groundwater δ 15 N‐ values declined from the upland edge through the riparian zone and were lowest near the stream. Groundwater δ 15 N‐ values increased for low electron donor (dissolved organic carbon) to acceptor ratios but decreased beyond a change point in ratios. Groundwater δ 15 N‐ values were particularly low for the riparian milldam site subjected to road‐salt salinization. We attributed these N isotopic trends to suppression of denitrification, occurrence of dissimilatory nitrate reduction to ammonium (DNRA), and/or effects of road salt salinization. Groundwater δ 15 N‐ can provide valuable insights into process mechanisms and can serve as “imprints” of anthropogenic activities and legacies.