Fluxes of CO2, CH4 and N2O from drained coniferous forests on organic soils

Fluxes of CO2, CH4 and N2O were measured during two to three years at four sites, located within an area of 9 km2 in southern Sweden, using dark static chamber techniques. Three of the sites were drained coniferous forests on moist organic soils that differed in forest productivity and tree species. The fourth site was an undrained tall sedge mire. Although the drained sites were all moist, with average groundwater levels between 17 and 27 cm below the soil surface, the mean annual dark forest floor CO2 release rate was significantly higher at the drained sites, (0.9-1.9 kg m−2 y−1) than at the undrained mire site (0.8 to 1.2 kg m−2 y−1). CH4 emissions were significantly lower from the drained sites than from the undrained mire (0.0 to 1.6 g m−2 y−1, compared to 10.6 to 12.2 g m−2 y−1), while N2O emissions were significantly lower from the undrained site than from the drained sites (20 to 30 mg m−2 y−1, compared to 30 to 90 mg m−2 y−1). There were no clear effects of site productivity or tree species on the soil fluxes of any of the gases. The annual net primary production of the forests was modeled. All drained sites were net sinks, while the undrained mire was a net source of greenhouse gases. The estimated net greenhouse gas exchange of the drained sites was correlated with productivity: the most productive site was the largest net sink and the least productive the smallest net sink for greenhouse gases. The results indicate that, to mitigate the increase of atmospheric greenhouse gases, drained forest sites, which have been unsuccessfully drained or rewetted due to subsidence, should be managed in a way that keeps the groundwater level at a steady state.