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

[摘要] Boreal upland forests are generally considered methane (CH 4 ) sinks dueto the predominance of CH 4 oxidizing bacteria over the methanogenicarchaea. However, boreal upland forests can temporarily act as CH 4 sources during wet seasons or years. From a landscape perspective and inannual terms, this source can be significant as weather conditions may causeflooding, which can last a considerable proportion of the active season andbecause often, the forest coverage within a typical boreal catchment is muchhigher than that of wetlands. Processes and conditions which change mineralsoils from acting as a weak sink to a strong source are not well understood.We measured soil CH 4 fluxes from 20 different points from regularlyirrigated and control plots during two growing seasons. We also estimatedpotential CH 4 production and oxidation rates in different soil layersand performed a laboratory experiment, where soil microcosms were subjectedto different moisture levels and glucose addition simulating the freshlabile carbon (C) source from root exudates. The aim was to find the keycontrolling factors and conditions for boreal upland soil CH 4 production. Probably due to long dry periods in both summers, we did notfind occasions of CH 4 production following the excess irrigation, withone exception in July 2019 with emission of 18 200  µ g CH 4  m −2  h −1 . Otherwise, the soil was always a CH 4 sink (medianCH 4 uptake rate of 260–290 and 150–170  µ g CH 4  m −2  h −1 , in control and irrigated plots, respectively). The median soilCH 4 uptake rates at the irrigated plot were 88 % and 50 % lowerthan at the control plot in 2018 and 2019, respectively. Potential CH 4 production rates were highest in the organic layer (0.2–0.6 nmol CH 4  g −1  d −1 ), but some production was also observed in the leachinglayer, whereas in other soil layers, the rates were negligible. PotentialCH 4 oxidation rates varied mainly within 10–40 nmol CH 4  g −1  d −1 , except in deep soil and the organic layer in 2019, where potentialoxidation rates were almost zero. The laboratory experiment revealed thathigh soil moisture alone does not turn upland forest soil into a CH 4 source. However, a simple C source, e.g., substrates coming from rootexudates with high moisture, switched the soil into a CH 4 source. Ourunique study provides new insights into the processes and controllingfactors on CH 4 production and oxidation, and the resulting net efflux thatshould be incorporated in process models describing global CH 4 cycling.

[学科分类] 大气科学