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Mitigation of nitrous oxide emissions in spray‐irrigated grazed grassland by treating the soil with dicyandiamide, a nitrification inhibitor

Abstract. Nitrous oxide (N2O) from animal excreta in grazed pasture systems makes up a significant component (c. 10%) of New Zealand's total greenhouse gas inventory. We report an effective method to decrease N2O emissions from animal urine patches by treating the soil with the nitrification inhibitor dicyandiamide (DCD), in a simulated grazed dairy pasture system under spray irrigation. The soil was a free‐draining Lismore stony silt loam (Udic Haplustept loamy skeletal) and the pasture was a mixture of perennial ryegrass (Lolium perenne) and white clover (Trifolium repens). By treating the soil with DCD, N2O emissions were decreased by 76% following urine application in the autumn, from 26.7 kg N2O‐N ha−1 without DCD to an average of 6.4 kg N2O‐N ha−1 with DCD over the 6‐month experimental period. N2O flux was decreased by 78% following urine application in the spring, from 18 kg N2O‐N ha−1 without DCD to 3.9 kg N2O‐N ha−1 with the application of DCD over the 3‐month period. A single application of DCD immediately after urine was sufficient to effectively mitigate N2O emissions from the urine. The results showed that repeated applications of DCD after urine application, or mixing DCD with urine, offered no advantage over a single application of DCD immediately after urine deposition.