The Effects of the Nitrification Inhibitor Dicyandiamide (DCD) in Streams

Abstract

A nitrification inhibitor, dicyandiamide (DCD), was introduced to New Zealand pastoral farming systems to reduce nitrate (NO3 -) leaching, nitrous oxide (N2O) emission, and to improve pasture yield. However, because of its soluble nature, the leaching of DCD into aquatic systems may occur and influence the natural nitrogen cycles by blocking nitrification and altering the availability of ammonium (NH4 +) and NO3 -, which may directly influence the uptake kinetics of algae and indirectly affect other chemical and biological conditions in stream systems. Two experiments based on stream microcosms, which containing stream water, sediments, rocks, and pumps, were conducted to explore the potential effects of DCD leaching on stream systems. The first experiment examined the impacts of the DCD presence at various concentrations on nitrogen cycling, algae biomass, and microbial activities, while the second experiment further tested the potential changes of these variables with the presence and absence of grazers after DCD application. Throughout these two experiments, DCD had had little to no effects on blocking nitrification, increasing the concentration of NH4 + and phosphate (PO4 3-), reducing NO3 - availability, stimulating or limiting algae biomass, or changing the activity of heterotrophic microbes. The low nitrification rate under these microcosm conditions, possibly caused by the high algae and microbial activities that out-competed the nitrifying bacteria for NH4 +, the extremely low NO3 - concentration after day 2 that made the NH4 + become the only nitrogen source for all organisms, and light inhibition effects, may explain the inefficacy of DCD. The degradation of DCD was significantly higher in Experiment 2 (half-life of 12 days at the mean temperature of 25°C) than that in Experiment 1 (half-life of 15.2 to 23.4 days at the mean temperature of 20°C), which may be possibly caused by the increased temperature. The colorimetric DCD measurement method might have be incorrect when dealing with low concentration samples with the threshold of at least 200 μg/L. The lack of significant differences of both algae abundance and microbial activities between control and DCD-treated groups in both experiments may indicate that DCD did not have toxic effects on algae and heterotrophic microbes. However, DCD may be toxic to snail (Potamopyrgus antipodarum) and its toxicity is possibly independent of the initial concentration.