Greenhouse gas exchange of rewetted bog peat extraction sites and a Sphagnum cultivation site in northwest Germany

During the last decades an increasing area of drained peatlands has been rewetted. Especially in Germany, rewetting is the principal treatment on cutover sites when peat extraction is finished. The objectives are bog restoration and the reduction of greenhouse gas (GHG) emissions. The first sites were rewetted in the 1980s. Thus, there is a good opportunity to study long-term effects of rewetting on greenhouse gas exchange, which has not been done so far on temperate cutover peatlands. Moreover, Sphagnum cultivating may become a new way to use cutover peatlands and agriculturally used peatlands as it permits the economical use of bogs under wet conditions. The climate impact of such measures has not been studied yet. We conducted a field study on the exchange of carbon dioxide, methane and nitrous oxide at three rewetted sites with a gradient from dry to wet conditions and at a Sphagnum cultivation site in NW Germany over the course of more than 2 years. Gas fluxes were measured using transparent and opaque closed chambers. The ecosystem respiration (CO2) and the net ecosystem exchange (CO2) were modelled at a high temporal resolution. Measured and modelled values fit very well together. Annually cumulated gas flux rates, net ecosystem carbon balances (NECB) and global warming potential (GWP) balances were determined. The annual net ecosystem exchange (CO2) varied strongly at the rewetted sites (from -201.7 +/- 126.8 to 29.7 +/- 112.7 gCO(2)-C m(-2) a(-1)) due to differing weather conditions, water levels and vegetation. The Sphagnum cultivation site was a sink of CO2 (-118.8 +/- 48.1 and -78.6 +/- 39.8 gCO(2)Cm(-2) a(-1)). The annual CH4 balances ranged between 16.2 +/- 2.2 and 24.2 +/- 5.0 gCH(4)-C-2 a(-1) at two inundated sites, while one rewetted site with a comparatively low water level and the Sphagnum farming site show CH4 fluxes close to 0. The net N2O fluxes were low and not significantly different between the four sites. The annual NECB was between -185.5 +/- 126.9 and 49.9 +/- 112.8 gCO(2)-Cm(-2)a(-1) at the rewetted sites and -115.8 +/- 48.1 and -77 +/- 39.8 gCO(2)Cm(-2)a(-1) at the Sphagnum cultivating site. The annual GWP100 balances ranged from -280.5 +/- 465.2 to 644.5 +/- 413.6 gCO(2)-eq.m(-2) a(-1) at the rewetted sites. In contrast, the Sphagnum farming site had a cooling impact on the climate in both years (-356.8 +/- 176.5 and -234.9 +/- 145.9 gCO(2)Cm(-2)a(-1)). If the carbon exported through the harvest of the Sphagnum biomass and the additional CO2 emission from the decay of the organic material is considered, the NECB and GWP100 balances are near neutral. Peat mining sites are likely to become net carbon sinks and a peat accumulating ("growing") peatland within 30 years of rewetting, but the GWP100 balance may still be positive. A recommended measure for rewetting is to achieve a water level of a few centimetres below ground. Sphagnum farming is a climate-friendly alternative to conventional commercial use of bogs. A year-round constant water level of a few centimetres below ground level should be maintained.