CH₄ and N₂O dynamics in the boreal forest–mire ecotone

Abstract. In spite of advances in greenhouse gas research, the spatiotemporal CH₄ and N₂O dynamics of boreal landscapes remain challenging, e.g., we need clarification of whether forest–mire transitions are occasional hotspots of landscape CH₄ and N₂O emissions during exceptionally high and low ground water level events.

In our study, we tested the differences and drivers of CH₄ and N₂O dynamics of forest/mire types in field conditions along the soil moisture gradient of the forest–mire ecotone. Soils changed from Podzols to Histosols and ground water rose downslope from a depth of 10 m in upland sites to 0.1 m in mires. Yearly meteorological conditions changed from being exceptionally wet to typical and exceptionally dry for the local climate. The median fluxes measured with a static chamber technique varied from −51 to 586 μg m⁻² h⁻¹ for CH₄ and from 0 to 6 μg m⁻² h⁻¹ for N₂O between forest and mire types throughout the entire wet–dry period.

In spite of the highly dynamic soil water fluctuations in carbon rich soils in forest–mire transitions, there were no large peak emissions in CH₄ and N₂O fluxes and the flux rates changed minimally between years. Methane uptake was significantly lower in poorly drained transitions than in the well-drained uplands. Water-saturated mires showed large CH₄ emissions, which were reduced entirely during the exceptional summer drought period. Near-zero N₂O fluxes did not differ significantly between the forest and mire types probably due to their low nitrification potential. When upscaling boreal landscapes, pristine forest–mire transitions should be regarded as CH₄ sinks and minor N₂O sources instead of CH₄ and N₂O emission hotspots.