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

A micrometeorological method is proposed for measuring the nitrous oxide (N2O) 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 N2O 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 N2O 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 N2O 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 (70 kg impregnated ewes), the stable boundary layer concentration increases of N2O and CO2 were highly correlated (0.080±0.008 ppb/ppm, r2=0.83, n=201, 3 min averages). The CO2 and N2O emission rates averaged 130±38 μg C m−2 s−1 and 24±5 ng N m−2 s−1, respectively, for a soil temperature of 19 °C. For synthetic-urine patches and untreated (but previously grazed) areas, soil cover/chamber measurements yielded N2O 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 N2O emissions were from urine patches, and emphasised the challenge of integrating small-scale emission measurements.