Controls for ecosystem methane exchange are time-scale specifc and shift during the growing season of a temperate fen

Wetlands are the largest natural sources for atmospheric methane (CH₄). In wetlands with permanent shallow inundation, the seasonal variation of CH₄ exchange is mainly controlled by temperature and phenology. In addition, ecosystem CH₄ exchange varies considerably on smaller temporal scales such as days or weeks. Several single processes that control CH₄ emissions on the local soil-plant-atmosphere continuum are well investigated, but their interaction on ecosystem level is not well understood yet. We applied wavelet analysis to a quasi-continuous Eddy Covariance CH₄ flux time series to describe the temporal variation of ecosystem CH₄ exchange within the growing season of a permanently inundated temperate fen. Moreover, we addressed time scale-specific controls and investigated whether their impact changes during the course of the growing season.

On large time scales of two weeks to three months, temperature explained most of the variation in ecosystem CH₄ exchange. In general, the temperature in the shallow water column had the largest impact as explanatory variable, however, air temperature and soil temperature became increasingly important as explanatory variables when water level dropped slightly up to June. The diurnal variation of ecosystem CH₄ exchange shifted during the course of the growing season: During a short time period at the end of April, plant activity (expressed by canopy photosynthesis) caused a diurnal variation of ecosystem CH₄ exchange with peak time around noon. In the following weeks, the daily cycle of convective mixing within the water column (expressed by the water temperature gradient) gradually gained importance and caused high night-time CH₄ emissions, thereby levelling off the diurnal CH₄ emission pattern. Moreover, shear-induced turbulence caused short-term fluctuations of ecosystem CH₄ exchange on time scales up to two hours.

Our study highlights the need for multi-scale approaches that consider the non-stationarity of the underlying processes to adequately describe the complexity of ecosystem CH₄ exchange. Moreover, we show that CH₄ release processes such as convective mixing of the water column which have been mainly considered for aquatic ecosystems might also be of importance in shallowly flooded terrestrial ecosystems.