Measurement of methane and carbon dioxide fluxes from peatland ecosystems by the conditional-sampling technique

Abstract

The conditional sampling method was used to measure methane (CH4) and carbon dioxide (CO2) fluxes from peatlands in northern Scotland. Preliminary data were obtained using a simple system. Subsequent automation made continuous flux-measurements possible. Observed CH4 fluxes were in the range −70 to +110 μmol m−2 h−1 with a mean flux of 23 μmol m−2 h−1. Peak photosynthetic CO2 fluxes were in the range −10 to −30 mmol m−2 h−1. Nocturnal respiration ranged from 0 to +10 mmol m−2 h−1. The conditional sampling observations showed reasonable agreement with measurements of flux by eddy-covariance, gradient and aircraft methods. Error analyses and laboratory tests were conducted to determine the precision of the flux-measurement system. The dominant error was associated with the determination of the mixing-ratio difference in conditionally-sampled updraught and downdraught air. The standard error of the difference for CH4 was typically 0.15 ng g−1 (0.3 parts in 109 (p.p.b.)) using a careful high-repetition sampling strategy with a modified gas chromatographic/flame ionization detector system. Under typical daytime atmospheric conditions this corresponded to a standard error in the flux measurement of 10 μmol m−2 h−1, which is consistent with field observations. The empirical beta factor in the conditional sampling equation was found to be insensitive to changes in turbulence intensity and atmospheric stability. Simple upscaling models were used to estimate annual carbon-fluxes to Great Britain's peatlands of −0.5 Mt of carbon in the form of CO2 and 18 kt in the form of methane.