Measuring nitrous oxide emission rate from grazed pasture using Fourier-transform infrared spectroscopy in the noctumal boundary layer

A micrometeorological method is proposed for measuring the nitrous oxide (N₂0) emission rate from grazed pasture. The method was employed at night and used open-path Fourier-transform infrared spectroscopy (FTIR) to simultaneously monitor concentrations of N₂0 and carbon dioxide (CO₂) at a height of 3 m in the stable boundary layer. A concentration measurement precision of 1% for 3 min averages was achieved over a 97 m long, open-air absorption path. During calm and clear nights, the formation of a shallow near-surface inversion layer effectively trapped surface gas emissions and led to a build-up of N₂0 and CO₂ concentrations near ground level. The ratio of these concentrations was combined with the more readily measured CO₂ emission rate to calculate an area-integrated N₂0 emission rate. The method was tested at Hollymount Farm, Springston, New Zealand (43.4°S, 172.3°E, 11 m above sea level) on an early autumn evening. For the Lismore silt loam, soil with ryegrass (Lolium perenne L.) pasture grazed by 10 sheep ha^-1 (70kg impregnated ewes), the stable boundary layer concentration increases of N₂0 and CO₂ were highly correlated (0.080±0.008 ppb/ppm, r² = 0.83, n = 201, 3 min averages). The CO₂ and N₂0 emission rates averaged 130 ± 38 μg C m^-2 s^-1 and 24 ± 5 ng Nm^-2s^-1, respectively, for a soil temperature of 19°C. For synthetic-urine patches and untreated (but previously grazed) areas, soil cover/chamber measurements yielded N₂0 emission data of 5-24 and 0-6 ng N m^-2 s^-1, respectively. This illustrated the value of chambers for determining spatial variability, suggested most of the N₂0 emissions were from urine patches, and emphasised the challenge of integrating small-scale emission measurements.