Volume 13, issue 4 | Copyright

Special issue: OzFlux: a network for the study of ecosystem carbon and water...

Biogeosciences, 13, 1309-1327, 2016
https://doi.org/10.5194/bg-13-1309-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 02 Mar 2016

Research article | 02 Mar 2016

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

Johannes Laubach1, Matti Barthel1,2, Anitra Fraser1, John E. Hunt1, and David W. T. Griffith3 Johannes Laubach et al.
  • 1Landcare Research, P.O. Box 69040, Lincoln 7640, New Zealand
  • 2Department of Environmental Systems Science, ETH Zürich, 8092 Zürich, Switzerland
  • 3School of Chemistry, University of Wollongong, Wollongong NSW 2522, Australia

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)nmolCH4m−2s−1 and 0.38 (±0.018)nmolN2Om−2s−1. During the same year, mean emission rates at the irrigated, fertilised and rotationally grazed site were 8.9 (±0.79)nmolCH4m−2s−1 and 0.58 (±0.020)nmolN2Om−2s−1. At this site, the N2O emissions amounted to 1.21 (±0.15)% of the nitrogen inputs from animal excreta and fertiliser application.

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We continuously measured CH4 and N2O fluxes on two pastures that differed with respect to management. Two micrometeorological ratio methods were used; one was more suitable for daytime and the other for night-time. Over a year, both methods indicated both sites as net sources of CH4 and N2O, similar to other managed grasslands. At the irrigated, fertilised and rotationally grazed site, the N2O emissions were 1.21 (±0.15) % of the nitrogen inputs from animal excreta and fertiliser application.
We continuously measured CH4 and N2O fluxes on two pastures that differed with respect to...
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