The effect of precipitation and application rate on dicyandiamide persistence and efficiency in two Irish grassland soils

The nitrification inhibitor dicyandiamide (DCD) has had variable success in reducing nitrate (NO3-) leaching and nitrous oxide (N2O) emissions from soils receiving nitrogen (N) fertilizers. Factors such as soil type, temperature and moisture have been linked to the variable efficacy of DCD. As DCD is water soluble, it can be leached from the rooting zone where it is intended to inhibit nitrification. Intact soil columns (15 cm diameter by 35 cm long) were taken from luvic gleysol and haplic cambisol grassland sites and placed in growth chambers. DCD was applied at 15 or 30 kg DCD/ha, with high or low precipitation. Leaching of DCD, mineral N and the residual soil DCD concentrations were determined over 8 weeks high precipitation increased DCD in leachate and decreased recovery in soil. A soil x DCD rate interaction was detected for the DCD unaccounted (proxy for degraded DCD). In the cambisol, degradation of DCD was high (circa 81%) and unaffected by DCD rate. In contrast, DCD degradation in the gleysol was lower and differentially affected by rate, 67 and 46% for the 15 and 30 kg/ha treatments, respectively. Variation in DCD degradation rates between soils may be related to differences in organic matter content and associated microbiological activity. Variable degradation rates of DCD in soil, unrelated to temperature or moisture, may contribute to changing DCD efficacy. Soil properties should be considered when tailoring DCD strategies for improving nitrogen use efficiency and crop yields, through the reduction of reactive nitrogen loss.