Effect of amending cattle urine with dicyandiamide on soil nitrogen dynamics and leaching of urinary-nitrogen

Oral administration of dicyandiamide (DCD) to grazing ruminants for excretion in urine represents an alternative delivery technique to conventional broadcast application of DCD to reduce urinary nitrogen (N) losses from grazed pastures. A field lysimeter trial and an allied plot mowing trial were conducted to examine the effects of intermixing DCD in cattle urine on soil and pasture N dynamics and leaching losses in a free-draining pumice soil. DCD was either intermixed with urine to achieve equivalent application rates of 10, 30 or 60 kg DCD haˉ¹, or surface broadcasted as a spray solution (10 or 30 kg DCD haˉ¹) onto the soil surface following urine application and compared to controls (urine and nil-controls without DCD). A single application of ¹⁵N-labelled artificial cattle urine (equivalent to 600 kg N haˉ¹) and corresponding DCD treatments were applied in late autumn and monitored over the following 300 days. DCD altered the partitioning of the applied urine-¹⁵N by increasing plant uptake of urinary-N by 32–60% and decreasing urine-¹⁵N in leachate, which was primarily influenced by the amount of DCD applied. The method of DCD application had no significant effect on leaching of any N constituent, except for ammonium-N, which was higher in the intermixed relative to the spray DCD treatments (26 vs. 18 kg N haˉ¹, respectively; P < 0.05). The total amount of nitrate-N leached was reduced from 217 kg N haˉ¹in the urine-control to 143, 80 and 61 kg N haˉ¹ (P < 0.05) with increasing rates of DCD application of 10, 30 and 60 kg haˉ¹, respectively. Application of DCD also significantly (P < 0.05) decreased the total amounts of ammonium-N and dissolved organic-N (DON) leached, but led to leaching losses of DCD-N. Rapid and large leaching losses of DCD down the soil profile led to spatial separation from the ammonium-N retained in the surface layer. Leaching of DCD (below 600 mm) at 55–69% of that applied represented an important leachable organic-N source (equivalent to 4–27 kg N haˉ¹), and reduced the overall efficacy of DCD in decreasing total N leaching losses. The total N leaching losses from all measured N fractions were 332 kg N haˉ¹ in the urine-control compared to 236, 157 and 154 kg N haˉ¹ at DCD application rates of 10, 30 and 60 kg haˉ¹, respectively. This study highlights the potential benefit from delivering DCD in cattle urine to reduce urinary-N leaching losses, with the most effective targeted single application rate being 30 kg haˉ¹ under the experimental conditions of this study