Factors affecting the degradation of the nitrification inhibitor dicyandiamide in a lowland Canterbury stream

Abstract

Dicyandiamide (DCD) is an effective nitrification inhibitor and can reduce nitrate (NO₃⁻) leaching losses and nitrous oxide (N₂O) gas emissions from soil. In New Zealand DCD was primarily used on dairy farms in the aim of reducing the environmental impact associated with NO₃⁻ leaching from dairy cow urine patches which can load the soil with up to 1000 kg N ha⁻¹. In soil, DCD is known to be degraded by soil microorganisms, but it also has the potential to be leached. As no literature exists on the degradation of DCD in waterways the purpose of this research was to investigate DCD degradation in a lowland Canterbury stream, and to identify factors which affect DCD degradation in waterways. This research involved a laboratory based mesocosm experiment comprising of four treatments, with three replicates of each, to simulate different waterway environments. The four treatments were; aerated stream water, aerated stream water with an undisturbed sediment base, aerated stream water containing plant material with an undisturbed sediment base, and anaerobic stream water with an undisturbed sediment base. Water, sediment, and plant material collected from the LII stream near Lincoln, New Zealand, was used for the experiment. Each mesocosm contained a depth of 75 cm of stream water with the concentrations of ammonium (NH₄⁺) and DCD, which were added to each mesocosm at a concentration of 1 mg L⁻¹, being monitored over time. Over the 40 day experiment DCD was not degraded in the aerated stream water treatment. In the three sediment containing treatments DCD was degraded with degradation occurring most rapidly in the aerated stream water with a sediment base treatment in which 25.8% of the added DCD being degraded, corresponding to a DCD half-life of 99 days. With the addition of plant material in the aerated stream water containing plant material with a sediment base treatment DCD degradation was slightly lower with 23.0% of the added DCD being degraded, corresponding to a DCD half-life of 139 days. This slightly slower degradation rate may be attributed to decomposing plant material providing an alternative, more accessable, source of nitrogen (N) to sediment microbes, or the accumulation of plant material detritus on the sediment surface may have reduced the incidence of DCD contacting sediment micoorganisms. Anaerobic conditions caused DCD degradation to be reduced to 20.4% of added DCD over 40 days, corresponding to a DCD half-life of 175 days. However, an artefact occurred in this treatment as anaerobic conditions were not constantly maintained due to ongoing photosynthetic activity in the mesocosms. The temperature (12.3-19.4˚C) and pH (7.4-9.5) range the treatments were subjected to in this experiment did not affect DCD degradation. In the experiment all of the added NH₄⁺-N was nitrified or removed from the stream water in each treatment within 10-25 days of addition. Therefore, it can be concluded that at concentrations of up to 1 mg L⁻¹ DCD does not inhibit nitrification in waterways and will not cause an accumulation of NH₃ .