Effects of DCD addition to slurry on nitrate leaching in sandy soils

The effects of the addition of the nitrification inhibitor dicyandiamide (DCD) to cattle slurry, applied in autumn to an arable sandy soil, were investigated in a three-year field experiment in the Netherlands. Treatments included application of slurry with DCD in November and December, application of slurry without DCD in November, December and January, and an untreated (slurry or DCD) control. Degradation of DCD, changes in mineral nitrogen in the soil, and leaching of nitrate and DCD were measured. Degradation of DCD in the topsoil was complete in May after application in the autumn before. However, at a depth of 90 cm, DCD was found three months after application. DCD remained present in leachate sampled at this depth for more than one year after application. Most probably it was then leached to the groundwater. Application of DCD considerably delayed nitrification. It had a large effect on the vertical distribution of mineral nitrogen in spring; more mineral nitrogen was found in the 0-40 cm soil layer and less in the 40-100 cm soil layer. However, neither time of application of the slurry nor addition of DCD had a significant effect on nitrate leaching.