Combining two complementary micrometeorological 1 methods to measure CH 4 and N 2 O fluxes over pasture

11 New Zealand’s largest industrial sector is pastoral agriculture, giving rise to a large fraction of 12 the country’s emissions of methane (CH4) and nitrous oxide (N2O). We designed a system to 13 continuously measure CH4 and N2O fluxes at the field scale on two adjacent pastures that 14 differed with respect to management. At the core of this system was a closed-cell Fourier15 transform infrared spectrometer (FTIR), measuring the mole fractions of CH4, N2O and 16 carbon dioxide (CO2) at two heights at each site. In parallel, CO2 fluxes were measured using 17 eddy-covariance instrumentation. We applied two different micrometeorological ratio 18 methods to infer the CH4 and N2O fluxes from their respective mole fractions and the CO2 19 fluxes. The first is a variant of the flux-gradient method, where it is assumed that the turbulent 20 diffusivities of CH4 and N2O equal that of CO2. This method was reliable when the CO2 21 mole-fraction difference between heights was at least 4 times greater than the FTIR’s 22 resolution of differences. For the second method, the temporal increases of mole fractions in 23 the stable nocturnal boundary layer, which are correlated for concurrently-emitted gases, are 24 used to infer the unknown fluxes of CH4 and N2O from the known flux of CO2. This method 25 was sensitive to “contamination” from trace gas sources other than the pasture of interest and 26 therefore required careful filtering. With both methods combined, estimates of mean daily 27 CH4 and N2O fluxes were obtained for 56 % of days at one site and 73 % at the other. Both 28 methods indicated both sites as net sources of CH4 and N2O. Mean emission rates for one year 29