02 Sep 2021

02 Sep 2021

Review status: this preprint is currently under review for the journal BG.

Excess soil moisture and fresh carbon input are prerequisites for methane production in podzolic soil

Mika Korkiakoski1, Tiia Määttä2, Krista Peltoniemi2, Timo Penttilä2, and Annalea Lohila1,3 Mika Korkiakoski et al.
  • 1Institute for Atmospheric and Earth System Research/Physics (INAR), Faculty of Science, P.O. Box 68, 00014 University of Helsinki, Helsinki, Finland
  • 2Natural Resources Institute Finland (Luke), Latokartanonkaari 9, 00790 Helsinki, Finland
  • 3Finnish Meteorological Institute, P.O. Box 503, FI-00101 Helsinki, Finland

Abstract. Boreal upland forests are generally considered methane (CH4) sinks due to the predominance of CH4 oxidising bacteria over the methanogenic archaea. However, boreal upland forests can temporarily act as CH4 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 CH4 fluxes from 20 different points from regularly irrigated and control plots during two growing seasons. We also estimated potential CH4 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 CH4 production. Probably due to long dry periods in both summers, we did not find occasions of CH4 production following the excess irrigation, with one exception in July 2019 with emission of 18200 μg CH4 m−2 h−1. Otherwise, the soil was always a CH4 sink (median CH4 uptake rate of 260–290 and 150–170 μg CH4 m−2 h−1, in control and irrigated plots, respectively). The median soil CH4 uptake rates at the irrigated plot were 88 % and 50 % lower than at the control plot in 2018 and 2019, respectively. Potential CH4 production rates were highest in the organic layer (0.2–0.6 nmol CH4 g−1 d−1), but some production was also observed in the leaching layer, whereas in other soil layers, the rates were negligible. Potential CH4 oxidation rates varied mainly within 10–40 nmol CH4 g−1 d−1, 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 CH4 source. However, a simple C source, e.g. substrates coming from root exudates with high moisture switched the soil into a CH4 source. Our unique study provides new insights into the processes and controlling factors on CH4 production and oxidation and resulting net efflux, that should be incorporated in process models describing global CH4 cycling.

Mika Korkiakoski et al.

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Mika Korkiakoski et al.

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Excess soil moisture and fresh carbon input are prerequisites for methane production in podzolic soil Mika Korkiakoski, Tiia Määttä; Krista Peltoniemi; Timo Penttilä; Annalea Lohila

Mika Korkiakoski et al.


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Short summary
We measured CH4 fluxes and production and oxidation potentials from irrigated and non-irrigated podzolic soil in a boreal forest. CH4 sink was smaller at the irrigated site, but did not cause CH4 emission without one exception. We also showed that under laboratory conditions, not only wet conditions but also fresh carbon are needed to make podzolic soil into a CH4 source. Our study provides important data for improving the process models describing the upland soil CH4 dynamics.