Effect of rate and form of dicyandiamide application on nitrate leaching and pasture production from a volcanic ash soil in the Waikato

Abstract

Animal urine has been identified as the main source of nitrate leaching from grazed pastures. The nitrification inhibitor dicyandiamide (DCD) can potentially reduce nitrate leaching losses and increase pasture utilisation of nitrogen (N) from urine. A mowing trial was established on a sandy loam (Typic Orthic Allophanic) soil in the Waikato region to examine the effect of liquid and granular forms of DCD, applied in May at rates of 0, 7, 14 and 28 kg ha‐1, on the fate of urea applied at 600 kg N ha‐1 in artificial urine. The DCD treatments increased pasture production over 211 days by an average of 15% (P < 0.001) and there was no significant effect of rate or form of DCD. Soil sampling 76 days after treatment application showed greater amounts of ammonium remaining in the top 450 mm of soil in the DCD treatments (average ammonium‐N for all DCD treatments of 93 kg N ha‐1 compared with 43 kg N ha‐1 for the urine‐only treatment). Total inorganic N in the top 600 mm of soil at the final (118 day) sampling showed a linear response to DCD application rate (113–190 kg ha‐1 from lowest to highest rate; P < 0.05) and the liquid DCD tended to retain more inorganic‐N than the granular DCD (179 and 131 kg ha‐1, respectively; P < 0.1). Averaged across all DCD treatments, 37% of the applied DCD was measured in the top 450 mm of soil at the day 48 sampling and most of that DCD was found below 150 mm. Leaching of N was measured using ceramic cup samplers at 600 mm depth in the 14 kg DCD ha‐1 treatments and showed that DCD reduced nitrate leaching by 24% from 275 to 208 kg ha‐1 (P < 0.01), with no significant effect of DCD form. Thus, DCD in both liquid and granular forms proved effective in reducing nitrate leaching from urine, indicating its potential for N loss mitigation in grazed pastures.