Nitric and nitrous oxide fluxes following bovine urine deposition to summer-grazed pasture

Nitric oxide (NO) and nitrogen dioxide (NO₂) emissions, referred to collectively as NO_x, cause decreases in methane and increases in tropospheric ozone. The net negative radiative forcing that ensues is dependent on NO_x flux magnitude, location and season. This study aimed to determine if urine–N rate affected NO_x or nitrous oxide (N₂O) fluxes following dairy cattle urine application to a temperate pasture soil under summer conditions (mean monthly rainfall 45 mm; mean air temperatures 17 °C, range 11–22 °C). An in situ experiment consisting of three treatments (control, bovine urine at 543 and 1086 kg N/ha) was replicated thrice in a randomised complete block design. Soil inorganic–N, pH and gas fluxes were measured over a 72 day summer period. Fluxes of NO–N in the control, 543 and 1086 kg N/ha treatments averaged 73, 328 and 341 µg NO–N/m²/h. Cumulative NO–N fluxes equalled 0.15% (± 0.03) and 0.20% (± 0.06) of the urine–N applied (± SD) in the 543 and 1086 kg N/ha treatments, respectively, with no effect of urine–N rate. Cumulative N₂O–N fluxes did not differ with urine–N rate equalling 0.14% (± 0.03) and 0.16% (±0.10) of the urine–N applied in the 543 and 1086 kg N/ha treatments, respectively. Under summer conditions, varying urine–N rate from 543–1086 kg N/ha had no significant effect on cumulative NO–N or N₂O–N when measured over 72 days.