The use of a nitrification inhibitor, dicyandiamide (DCD), to decrease nitrate leaching and nitrous oxide emissions in a simulated grazed and irrigated grassland

In grazed dairy pasture systems, a major source of NO 3 – leached and N 2 O emitted is the N returned in the urine from the grazing animal. The objective of this study was to use lysimeters to measure directly the effectiveness of a nitrification inhibitor, dicyandiamide (DCD), in decreasing NO 3 – leaching and N 2 O emissions from urine patches in a grazed dairy pasture under 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 ). The use of DCD decreased NO 3 – ‐N leaching by 76% for the urine N applied in the autumn, and by 42% for urine N applied in the spring, giving an annual average reduction of 59%. This would reduce the NO 3 – ‐N leaching loss in a grazed paddock from 118 to 46 kg N ha –1 yr –1 . The NO 3 – ‐N concentration in the drainage water would be reduced accordingly from 19.7 to 7.7 mg N L –1 , with the latter being below the drinking water guideline of 11.3 mg N L –1 . Total N 2 O emissions following two urine applications were reduced from 46 kg N 2 O‐N ha –1 without DCD to 8.5 kg N 2 O‐N with DCD, representing an 82% reduction. In addition to the environmental benefits, the use of DCD also increased herbage production by more than 30%, from 11 to 15 t ha –1 yr –1 . The use of DCD therefore has the potential to make dairy farming more environmentally sustainable by reducing NO 3 – leaching and N 2 O emissions.