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

Abstract

Nitric oxide (NO) and nitrogen dioxide (NO2) emissions, referred to collectively as NOx, cause decreases in methane and increases in tropospheric ozone. The net negative radiative forcing that ensues is dependent on NOx flux magnitude, location and season. This study aimed to determine if urine–N rate affected NOx 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.