Mosses as biofilters for ditch methane emissions from forestry drained peatlands

In drained peatland forests, drainage ditches cover ca. 3% of the area, but contribute to up to 100% of methane (CH₄) emission. The drained peat soils, in contrast, can act as a CH₄ sink especially under efficient drainage. Therefore, emissions from ditches will impact whether drained peatland is a net CH₄ sink or source. The net CH₄ flux is likely to be impacted by the conditions in the ditches such as the extent and type of plant cover and the time since drainage. In order to provide more accurate ditch CH₄ emission factors for national greenhouse gas (GHG) inventory, we examined the fluxes and the underlying CH₄ cycling processes in a nutrient rich peatland forest in Ränskälänkorpi, Southern Finland during May-October 2021. We compared the ecosystem-atmosphere CH₄ fluxes and their δ¹³C values from moss dominated and open water ditches. We determined CH₄ and CO₂ mixing ratios and their δ¹³C values in water and in sediment by gas chromatography and cavity ring-down spectroscopy (Picarro G2201-i), respectively. We also assessed the role of CH₄ as a carbon source for Sphagnum mosses growing in ditches by analyzing δ¹³C values in submerged and partly submerged Sphagnum using elemental analysis - isotope ratio mass spectrometry. We found that mean seasonal CH₄ emissions from moss dominated ditches were 90% lower than from open water surfaces. In this dry summer, moss-dominated ditches were occasionally net sinks of atmospheric CH₄. These results can be explained by CH₄ consuming microbes inhabiting surface water, moss layer or sediment below the moss layer and using CH₄ as a source of carbon and energy. Isotopic mass balance calculations accounting for the measured δ¹³C values of Sphagnum moss, dissolved CO₂ and CH₄ as well as fractionation against ¹³C during (mass-transfer-limited) moss CO₂ fixation indicated that 10-28% of carbon in ditch Sphagnum mosses potentially originated from oxidized CH₄. Ditch network maintenance, including removing mosses, is likely to decrease along with changing peatland forest management, e.g., continuous cover forestry. Our results suggest that ditches overgrown by mosses have potential to reduce CH₄ emissions from drained peatland forests and could serve as an additional GHG mitigation measure to management practices that maintain a continuous forest cover, attenuate the changes in water table level and thus reduce CH₄ emissions from peat soils.