By Marc Schallenberg
In many ways, the nitrification inhibitor DCD (dicyandiamide), described previously by Dr Bob Wilcock of NIWA, seems like the proverbial “silver bullet”. In some situations it reduces nitrate leaching and nitrous oxide emissions from dairy farms – two serious environmental issues resulting from the New Zealand dairy boom of the last 15-20 years. It has a low toxicity and breaks down relatively quickly (weeks to months) in the environment. No wonder it was increasingly being promoted for use on dairy pastures.
But to an environmental scientist like myself, the increasing use of DCD in the environment raises some potentially troubling questions. For example, DCD is soluble in water, so it seems likely that it could end up in places where it isn’t intended to be. Indeed, residues of the chemical have recently turned up in NZ milk. But my main concern was what would happen to beneficial natural nitrogen transformations in freshwaters if residues of DCD were to leach into wetlands, lakes and estuaries, where nitrification and denitrification are extremely important natural detoxifying and self-purifying processes.
To examine this, MSc student Ian Smith and I developed a two-pronged research project to find out 1) if DCD could be measured in surface waters of the lower Taieri Plain (Otago), where DCD is in use and if so, 2) does it have powerful effects on nitrogen cycling in waters, as it does on some dairy paddocks (Fig. 1).
Measurable residues of DCD turned up in most of the stream and drain sites sampled in the lower Tairei Plain. While we were only mildly surprised that it turned up in springtime (October and November), we were quite surprised to find it also in summertime (January), long past the usual spring application period for DCD. If DCD were to have a strong effect on denitrification in the aquatic environment we expected the concentrations of DCD in water to be correlated with the amount of ammonium in the water and with the ratio of ammonium:nitrate. The survey samples did indeed show this (Fig. 2), but this did not conclude DCD was active in the aquatic environment. We would also expect to see less nitrate relative to ammonium concentrations in runoff from DCD-treated paddocks.
So we decided to set up additional experiments to test whether DCD was altering nitrogen transformations in a freshwater system when spiked with range of DCD concentrations similar to those observed in the streams and drains of the lower Taieri Plain. We set up a closed wetland system in the laboratory, where inputs, losses and transformations of nitrogen could be measured and monitored over time. Sediments and water were retrieved from a wetland in the lower Taieri Plain and put into a large aquarium. Short lengths of drain pipe were then inserted through the water into the sediment, providing 21 sediment-water cores. DCD was added to some of the cores in different concentrations. Initially, the conditions in the cores favoured dentirification with a small but surprising increase in denitrification rates in the presence of DCD. Then conditions were altered (ammonium was added) to favour nitrification. From this point onwards, DCD strongly reduced the conversion of ammonium to nitrate in the cores, just as it does on some dairy paddocks (Fig. 3).
As a result, this study shows that the way farmers have been using DCD results in DCD residues entering freshwater ecosystems, where it can inhibit the important natural process of nitrification. However, by blocking the conversion of ammonium to nitrate in soils, the use of DCD in paddocks should decrease the total amount of nitrogen leaching to freshwaters from dairy activities. So the effects of DCD on nitrogen cycling demonstrated in this study should mainly be of concern in waters that receive substantial amounts of ammonium from other sources. These could be anoxic groundwater inputs, inputs of sewage or other industrial wastes, or where amonium production is already naturally high.
This study is the first to examine impacts of DCD on nitrogen cycling in aquatic ecosystems and its results support the current withdrawal of DCD from use at least until these effects can be studied in a variety of aquatic ecosystems and at larger scales.
Smith, I. & Schallenberg, M. (2013). Occurrence of the agricultural nitrification inhibitor, dicyandiamide, in surface waters and its effects on nitrogen dynamics in an experimental aquatic system. Agriculture, Ecosystems and Environment 164: 23-31.
Dr Marc Schallenberg is a Research Fellow in the Zoology Department at the University of Otago.