Occurrence of the agricultural nitrification inhibitor, dicyandiamide, in surface waters and its effects on nitrogen dynamics in an experimental aquatic system

Nitrification inhibitors are promoted to mitigate nitrate leaching and N2O emissions from grazed pastures. We hypothesized that the nitrification inhibitor, dicyandiamide (DCD), appears in surface waters in agricultural areas where it is used and that DCD perturbs nitrogen cycling in surface waters as it does in soils. We sampled 15 streams and drains in a coastal lowland agricultural catchment and found measurable concentrations of DCD in many of the surface waters, with a maximum measured concentration of approximately 1000 μg L−1. DCD concentrations were positively correlated with ammonium concentrations, the ratio of ammonium:nitrate, and electrical conductivity (salinity), while DCD concentrations were negatively correlated with dissolved oxygen concentrations. These results are consistent with DCD entering waterways via shallow groundwater seepage and having an inhibitory effect on in situ nitrification in the aquatic ecosystems.