Comparison of chamber and eddy covariance CH4 fluxes using a new “open path” gas analyzer

Traditional measurements of CH₄ fluxes are made with static chamber techniques and/or with closed-path eddy covariance (EC) systems. Although each approach has its advantages, they also pose some critical limitations: measurements based on static chamber are time consuming, do not capture the dynamics of CH₄ fluxes on continuous temporal scales due to the discontinuity of the sample collection, and can cover only a small soil surface. The development of a new “open-path” CH₄ analyzer (LI-7700 LI-COR Biosciences, Lincoln, NE USA) provides the frequency response required by the EC measures and overcomes the pre-mentioned problems. Our objective was to test the application of LI-7700 and compare the results with the fluxes based on static chamber for justifying our future long term measurements of CH₄. We measured the CH4 fluxes using a LI-7700 “open-path” gas analyzer in conjunction with a CSAT3 3-D sonic anemometer (Campbell Scientific Inc. CSI, UT, USA) and a LI-7550 (LI-COR Biosciences, Lincoln, NE) analyzer interface unit. Measurements from the chambers were performed using in situ closed-cover flux chambers following the methodology and fluxes calculation reported in: http://lter.kbs.msu.edu/protocols/23. We found a good agreement between the two techniques. EC CH₄ fluxes showed no consistent daily pattern with the values that ranged from 0.001 to -0.009 mg CH4 m^-2. Daily mean CH₄ density showed considerable day-to-day variation varied  from 15 to 1.7 ppm, with particular high variation during the summer period when there are more rainy days than the autumn/winter season.  We did not found any significant correlation between CH4 fluxes with soil water content or temperature. The new open-path CH₄ analyzer appeared to be reliable in locations that are not equipped with grid power. However, a necessity to conduct such comparison in sites characterized by higher level of CH₄ emission is a critical step.