Fate of the nitrification inhibitor dicyandiamide (DCD) sprayed on a grazed pasture: effect of rate and time of application

Abstract

The objectives of this study were to improve our understanding of seasonal variation in the biophysical disappearance of the nitrification inhibitor dicyandiamide (DCD) in soil and the key regulatory factors. Changes in DCD concentrations in the soil and plant canopy were measured following application to dairy-grazed pasture soil and non-grazed pasture soil. Treatments included two levels of DCD alone (10 and 20 kg ha–1) applied to non-grazed pasture field plots, and DCD (10 kg ha–1) applied with urine and with urea fertiliser. DCD (10 kg ha–1) was also applied in grazed farmlets following grazing. About 4–40% of the DCD applied was intercepted and stayed on the plant canopy from <6 and up to 16 days, depending on the subsequent timing and intensity of rainfall. In this poorly drained soil, <10% of applied DCD leached below 10 cm depth. Neither the level of DCD nor the N source had any significant effect on the half-life of DCD in soil. The half-life of DCD did vary with season, ranging from 7 to 13 days in March to November respectively, and showed a linear decrease with observed increase in soil temperature between 10.7 and 16.5°C. The results suggest that to maintain an optimum effective DCD concentration in soil, different DCD application rates and frequency may be required in different seasons.