Biotic and Abiotic Control Over Diurnal CH 4 Fluxes in a Temperate Transitional Poor Fen Ecosystem

To understand the variability of methane (CH 4 ) fluxes between a temperate mid-altitude Sphagnum- dominated peatland and the atmosphere, we monitored simultaneously eddy covariance, hydrometeorological and physical parameters between April 2019 and December 2021. The site was a CH 4 source for the atmosphere, with a cumulative emission of 23.9 ± 0.6 g C m −2 year −1 . At the interannual scale, deeper water table during vegetation growth periods resulted in lower CH 4 fluxes (FCH4), and reciprocally. Furthermore, the seasonal temperature variation in the anaerobic peat layer was a good predictor for FCH4. However, while the lowest temperatures occurred between December and February, the lowest FCH4 were observed between March and May, with around 30% of negative FCH4. Indeed, the fastest increase in temperature of the aerobic layer likely stimulated methanotrophy at the expense of methanogenesis. Negative FCH4, systematically observed at midday, were concurrent with high photon flux densities, latent heat fluxes and net negative ecosystem CO 2 exchanges, suggesting the control of photosynthesis over CH 4 oxidation. Moreover, our results highlighted marked diurnal cycles with FCH4 maximal at night and minimal at midday for all seasons. This diurnal cyclicity is in opposition to what is typically known for peatlands dominated by vascular plants. Physical parameters, such as soil surface temperature and sensible heat fluxes, likely contribute to this diurnal FCH4 cyclicity and require further investigation. Our study thus demonstrates that diurnal variations in FCH4 must be considered before upscaling to seasonal or annual cycles, along with the effect of vegetation on CH 4 transfer and oxidation processes.