Soil CO2, CH4 and N2O concentrations and fluxes in peatland forests are associated with water table level - implications of selection harvesting on soil emissions

Greenhouse gas (GHG) emissions from peatland forest soils are associated with ground water table (WT) level, which controls the vertical distribution of aerobic and anaerobic processes in soil. It has been suggested that transition from rotation forestry with ditch network maintenance (DNM) to selection harvesting would be a feasible alternative to reduce negative GHG and water impacts of peatland forestry as it raises WT and reduces aerobic decomposition of deep peat compared to mature forests. Transpiration from remaining trees would keep WT low enough for stand growth and natural regeneration without DNM. We measured vertical CO₂, CH₄, N₂O and O₂ concentration profiles in two peatland forests to provide insights on the controls of processes producing and consuming gases in the soil after harvest-induced hydrological change. The sites located on nutrient-rich peatland soils in Southern Finland, were dominated by Norway spruce, and parts of the sites were selection harvested. Selection harvesting raised WT by 14 cm relative to non-harvested controls, on average. All soil gas concentrations were associated with the proximity to the WT, but their patterns and in-soil fluxes were decoupled. CH₄ and CO₂ showed remarkable vertical concentration gradients, with very high values in the deepest layer due to low gas permeability of wet peat. However, CH₄ was efficiently consumed in the peat layers above the WT where it reached sub-atmospheric concentrations, indicating oxidation of CH₄ from both atmospheric and deeper origins. Soils maintained these functions after selection harvest. Surface peat contributed the most to soil-atmosphere CO₂ flux, but harvest treatment also modestly increased the source in deeper soil. No consistent differences were observed in N₂O emissions, which were the least associated with other gases. Based on our results, selection harvesting in drained nutrient-rich peatland forests without other hydrological measures limitedly reduces soil net emissions compared to non-harvested mature stands.