The effect of atmospheric turbulence and chamber deployment period on autochamber CO2 and CH4 flux measurements in an ombrotrophic peatland

[摘要] Accurate quantification of soil-atmosphere gas exchange is essential forunderstanding the magnitude and controls of greenhouse gas emissions. Weused an automatic, closed, dynamic chamber system to measure the fluxes ofCO₂ and CH₄ for several years at the ombrotrophic Mer Bleuepeatland near Ottawa, Canada and found that atmospheric turbulence andchamber deployment period had a considerable influence on the observed fluxrates. With a short deployment period of 2.5 min, CH₄ fluxexhibited strong diel patterns and both CH₄ and nighttime CO₂effluxes were highly and negatively correlated with ambient frictionvelocity as were the CO₂ concentration gradients in the top 20 cm ofpeat. This suggests winds were flushing the very porous and relatively drynear-surface peat layers and reducing the belowground gas concentrationgradient, which then led to flux underestimations owing to a decrease inturbulence inside the headspace during chamber deployment compared to theambient windy conditions. We found a 9 to 57% underestimate of the netbiological CH₄ flux at any time of day and a 13 to 21% underestimateof nighttime CO₂ effluxes in highly turbulent conditions. Conversely,there was evidence of an overestimation of ~ 100% of netbiological CH₄ and nighttime CO₂ fluxes in calm atmosphericconditions possibly due to enhanced near-surface gas concentration gradientby mixing of chamber headspace air by fans. These problems were resolved byextending the deployment period to 30 min. After 13 min of chamberclosure, the flux rate of CH₄ and nighttime CO₂ became constantand were not affected by turbulence thereafter, yielding a reliable estimateof the net biological fluxes. The measurement biases we observed likelyexist to some extent in all chamber flux measurements made on porous andaerated substrate, such as peatlands, organic soils in tundra and forests,and snow-covered surfaces, but would be difficult to detect unless highfrequency, semi-continuous observations were made.

[学科分类] 地球化学与岩石