Combining two complementary micrometeorological methods to measure CH₄ and N₂O fluxes over pasture

Abstract. New Zealand's largest industrial sector is pastoral agriculture, giving rise to a large fraction of the country's emissions of methane (CH₄) and nitrous oxide (N₂O). We designed a system to continuously measure CH₄ and N₂O fluxes at the field scale on two adjacent pastures that differed with respect to management. At the core of this system was a closed-cell Fourier transform infrared (FTIR) spectrometer, which measured the mole fractions of CH₄, N₂O and carbon dioxide (CO₂) at two heights at each site. In parallel, CO₂ fluxes were measured using eddy-covariance instrumentation. We applied two different micrometeorological ratio methods to infer the CH₄ and N₂O fluxes from their respective mole fractions and the CO₂ fluxes. The first is a variant of the flux-gradient method, where it is assumed that the turbulent diffusivities of CH₄ and N₂O equal that of CO₂. This method was reliable when the CO₂ mole-fraction difference between heights was at least 4 times greater than the FTIR's resolution of differences. For the second method, the temporal increases of mole fractions in the stable nocturnal boundary layer, which are correlated for concurrently emitted gases, are used to infer the unknown fluxes of CH₄ and N₂O from the known flux of CO₂. This method was sensitive to “contamination” from trace gas sources other than the pasture of interest and therefore required careful filtering. With both methods combined, estimates of mean daily CH₄ and N₂O fluxes were obtained for 56 % of days at one site and 73 % at the other. Both methods indicated both sites as net sources of CH₄ and N₂O. Mean emission rates for 1 year at the unfertilised, winter-grazed site were 8.9 (±0.79) nmol CH₄ m⁻² s⁻¹ and 0.38 (±0.018) nmol N₂O m⁻² s⁻¹. During the same year, mean emission rates at the irrigated, fertilised and rotationally grazed site were 8.9 (±0.79) nmol CH₄ m⁻² s⁻¹ and 0.58 (±0.020) nmol N₂O m⁻² s⁻¹. At this site, the N₂O emissions amounted to 1.21 (±0.15) % of the nitrogen inputs from animal excreta and fertiliser application.