Riparian groundwater nitrogen (N) isotopes reveal human imprints of dams and road salt salinization

Groundwater nitrate-N isotopes (δ15N-NO3-) 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 δ15N-NO3- for riparian zones affected by relict milldams and salinization. Contrary to natural conditions, groundwater δ15N-NO3- values declined from the upland edge through the riparian zone and were lowest near the stream. Groundwater δ15N-NO3- values increased for low electron donor (dissolved organic carbon and iron) to acceptor (NO3-) ratios but decreased beyond a specific change point. Groundwater δ15N-NO3- 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 road salt salinization. Groundwater δ15N-NO3- can provide valuable insights into process mechanisms and can serve as “imprints” of anthropogenic activities and legacies.