Measurements of CO2 exchange with an automated chamber system throughout the year: challenges in measuring night-time respiration on porous peat soil

[摘要] We built an automatic chamber system to measure greenhouse gas (GHG)exchange in forested peatland ecosystems. We aimed to build a systemrobust enough which would work throughout the year and could measurethrough a changing snowpack in addition to producing annual GHG fluxesby integrating the measurements without the need of using models. Thesystem worked rather well throughout the year, but it was not servicefree. Gap filling of data was still necessary.

We observed problems in carbon dioxide (CO₂) respiration flux estimationduring calm summer nights, when a CO₂ concentration gradientfrom soil/moss system to atmosphere builds up. Chambers greatlyoverestimated the night-time respiration. This was due to thedisturbance caused by the chamber to the soil-moss CO₂gradient and consequent initial pulse of CO₂ to the chamberheadspace. We tested different flux calculation and measurementmethods to solve this problem. The estimated flux was stronglydependent on (1) the starting point of the fit after closing the chamber, (2)the length of the fit, (3) the type of the fit (linear and polynomial), (4)the speed of the fan mixing the air inside the chamber, and (5) atmosphericturbulence (friction velocity, u*). The best fitting method (the most robust, least random variation) for respiration measurements on our sites waslinear fitting with the period of 120–240 s after chamberclosure. Furthermore, the fan should be adjusted to spin at minimumspeed to avoid the pulse-effect, but it should be kept on to ensuremixing. If night-time problems cannot be solved, emissions can beestimated using daytime data from opaque chambers.

[学科分类] 地球化学与岩石