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.
Highlights
Methane (CH4) is a greenhouse gas with a significant impact on the global climate
Based on our field and laboratory experiments, the main conclusion is that CH4 production from boreal upland forest soil cannot occur solely by prolonged wet conditions, but there has to be enough fresh carbon in the soil
We expect the possible CH4 production episodes to occur in late summer and autumn rather than in spring, even though the soil can be very wet after snowmelt
Summary
Methane (CH4) is a greenhouse gas with a significant impact on the global climate. CH4 increases the global temperatures by absorbing infrared radiation into its carbon-hydrogen bonds, resulting in a higher amount of heat energy within the atmosphere (e.g. Chai et al, 2016; Dlugokencky et al, 2011; Whalen, 2005). The dynamics behind soil CH4 sources and sinks depend on the ratio between CH4 production and oxidation and its transport from the soil to the atmosphere, all of which are affected by an extensive network of numerous biotic and abiotic variables. The interannual fluctuations in global and regional CH4 emissions 10 are influenced by so-far largely unknown variables, the investigation of which is essential for understanding the changing dynamics in the current and future CH4 budgets (Bousquet et al, 2006; Crill and Thornton, 2017; Dlugokencky et al, 2011; Fischer et al, 2008; Kirschke et al, 2013). The boreal zone in the northern hemisphere regularly presents large CH4 emissions due to the abundance of anoxic wetlands, but part is counterbalanced by high oxidation rates in boreal upland forests. The CH4 emission estimates from the boreal zone lie between 25 and 100 Tg yr−1, which combined with subarctic tundra environments 15 account for approximately 3–10 % of the global CH4 emissions (Olefeldt et al, 2013)
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