Methane exchange at the peatland forest floor – automatic chamber system exposes the dynamics of small fluxes

Abstract. We measured methane (CH₄) exchange rates with automatic chambers at the forest floor of a nutrient-rich drained peatland in 2011–2013. The fen, located in southern Finland, was drained for forestry in 1969 and the tree stand is now a mixture of Scots pine, Norway spruce, and pubescent birch. Our measurement system consisted of six transparent chambers and stainless steel frames, positioned on a number of different field and moss layer compositions. Gas concentrations were measured with an online cavity ring-down spectroscopy gas analyzer. Fluxes were calculated with both linear and exponential regression. The use of linear regression resulted in systematically smaller CH₄ fluxes by 10–45 % as compared to exponential regression. However, the use of exponential regression with small fluxes ( <  2.5 µg CH₄ m⁻² h⁻¹) typically resulted in anomalously large absolute fluxes and high hour-to-hour deviations. Therefore, we recommend that fluxes are initially calculated with linear regression to determine the threshold for low fluxes and that higher fluxes are then recalculated using exponential regression. The exponential flux was clearly affected by the length of the fitting period when this period was  <  190 s, but stabilized with longer periods. Thus, we also recommend the use of a fitting period of several minutes to stabilize the results and decrease the flux detection limit. There were clear seasonal dynamics in the CH₄ flux: the forest floor acted as a CH₄ sink particularly from early summer until the end of the year, while in late winter the flux was very small and fluctuated around zero. However, the magnitude of fluxes was relatively small throughout the year, ranging mainly from −130 to +100 µg CH₄ m⁻² h⁻¹. CH₄ emission peaks were observed occasionally, mostly in summer during heavy rainfall events. Diurnal variation, showing a lower CH₄ uptake rate during the daytime, was observed in all of the chambers, mainly in the summer and late spring, particularly in dry conditions. It was attributed more to changes in wind speed than air or soil temperature, which suggest that physical rather than biological phenomena are responsible for the observed variation. The annual net CH₄ exchange varied from −104 ± 30 to −505 ± 39 mg CH₄ m⁻² yr⁻¹ among the six chambers, with an average of −219 mg CH₄ m⁻² yr⁻¹ over the 2-year measurement period.