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

Journal Article


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 spectrometer (FTIR), measuring 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 60 % of days at one site and 77 % at the other. Both methods indicated both sites as net sources of CH4 and N2O. Mean emission rates for one year at the unfertilised, winter-grazed site were 8.2 (�� 0.91) nmol CH4 m-2 s-1 and 0.40 (�� 0.018) nmol N2O m-2 s-1. During the same year, mean emission rates at the irrigated, fertilised and rotationally-grazed site were 7.0 (�� 0.89) nmol CH4 m-2 s-1 and 0.57 (�� 0.019) nmol N2O m-2 s-1. At this site, the N2O emissions amounted to 1.19 (�� 0.15) % of the nitrogen inputs from animal excreta and fertiliser application.

Publication Date


  • 2015

Citation


  • Laubach, J., Barthel, M., Fraser, A., Hunt, J. E., & Griffith, D. W. T. (2015). Combining two complementary micrometeorological methods to measure CH4 and N2O fluxes over pasture. Biogeosciences Discussions, 12(18), 15245-15299. doi:10.5194/bgd-12-15245-2015

Scopus Eid


  • 2-s2.0-85010939058

Web Of Science Accession Number


Start Page


  • 15245

End Page


  • 15299

Volume


  • 12

Issue


  • 18

Place Of Publication


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 spectrometer (FTIR), measuring 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 60 % of days at one site and 77 % at the other. Both methods indicated both sites as net sources of CH4 and N2O. Mean emission rates for one year at the unfertilised, winter-grazed site were 8.2 (�� 0.91) nmol CH4 m-2 s-1 and 0.40 (�� 0.018) nmol N2O m-2 s-1. During the same year, mean emission rates at the irrigated, fertilised and rotationally-grazed site were 7.0 (�� 0.89) nmol CH4 m-2 s-1 and 0.57 (�� 0.019) nmol N2O m-2 s-1. At this site, the N2O emissions amounted to 1.19 (�� 0.15) % of the nitrogen inputs from animal excreta and fertiliser application.

Publication Date


  • 2015

Citation


  • Laubach, J., Barthel, M., Fraser, A., Hunt, J. E., & Griffith, D. W. T. (2015). Combining two complementary micrometeorological methods to measure CH4 and N2O fluxes over pasture. Biogeosciences Discussions, 12(18), 15245-15299. doi:10.5194/bgd-12-15245-2015

Scopus Eid


  • 2-s2.0-85010939058

Web Of Science Accession Number


Start Page


  • 15245

End Page


  • 15299

Volume


  • 12

Issue


  • 18

Place Of Publication