<p>Boreal upland forests are generally considered methane (CH<sub>4</sub>) sinks due to the predominance of CH<sub>4</sub> oxidising bacteria over the methanogenic archaea. However, boreal upland forests can temporarily act as CH<sub>4</sub> sources during wet seasons or years. From a landscape perspective and in annual terms, this source can be significant as weather conditions may cause flooding, which can last a considerable proportion of the active season and because often, the forest coverage within a typical boreal catchment is much higher than that of wetlands. Processes and conditions which change mineral soils from acting as a weak sink to a strong source are not well understood. We measured soil CH<sub>4</sub> fluxes from 20 different points from regularly irrigated and control plots during two growing seasons. We also estimated potential CH<sub>4</sub> production and oxidation rates in different soil layers and performed a laboratory experiment, where soil microcosms were subjected to different moisture levels and glucose addition simulating the fresh labile carbon (C) source from root exudates. The aim was to find the key controlling factors and conditions for boreal upland soil CH<sub>4</sub> production. Probably due to long dry periods in both summers, we did not find occasions of CH<sub>4</sub> production following the excess irrigation, with one exception in July 2019 with emission of 18200 μg CH<sub>4</sub> m<sup>−2</sup> h<sup>−1</sup>. Otherwise, the soil was always a CH<sub>4</sub> sink (median CH<sub>4</sub> uptake rate of 260–290 and 150–170 μg CH<sub>4</sub> m<sup>−2</sup> h<sup>−1</sup>, in control and irrigated plots, respectively). The median soil CH<sub>4</sub> uptake rates at the irrigated plot were 88 % and 50 % lower than at the control plot in 2018 and 2019, respectively. Potential CH<sub>4</sub> production rates were highest in the organic layer (0.2–0.6 nmol CH<sub>4</sub> g<sup>−1</sup> d<sup>−1</sup>), but some production was also observed in the leaching layer, whereas in other soil layers, the rates were negligible. Potential CH<sub>4</sub> oxidation rates varied mainly within 10–40 nmol CH<sub>4</sub> g<sup>−1</sup> d<sup>−1</sup>, except in deep soil and the organic layer in 2019, where potential oxidation rates were almost zero. The laboratory experiment revealed that high soil moisture alone does not turn upland forest soil into a CH<sub>4</sub> source. However, a simple C source, e.g. substrates coming from root exudates with high moisture switched the soil into a CH<sub>4</sub> source. Our unique study provides new insights into the processes and controlling factors on CH<sub>4</sub> production and oxidation and resulting net efflux, that should be incorporated in process models describing global CH<sub>4</sub> cycling.</p>