Greenhouse gas emissions from rewetted bog peat extraction sites and a <i>Sphagnum</i> cultivation site in Northwest Germany
Abstract. During the last three decades, an increasing area of drained peatlands was rewetted. This was done with the objective to convert these sites from sources back to sinks or, at least, to much smaller sources of greenhouse gases (GHG). However, available data is still scarce, especially on the long-term climatic effects of rewetting of temperate bogs. Moreover, first field trials are established for Sphagnum cultivating (paludiculture) on wet bog sites and an assessment of 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 more than two years. Gas fluxes were measured using transparent and opaque closed chambers. The ecosystem respiration (CO2) and the net ecosystem exchange (CO2) were modelled in high time resolution using automatically monitored climate data. Measured and modelled values fit very well together (R2 between 0.88 and 0.98). 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 g CO2-C m–2 a–1) due to different weather conditions, water level and vegetation. The Sphagnum cultivation site was a sink of CO2 (–118.8 ± 48.1 and −78.6 ± 39.8 g CO2-C m–2 a–1). The yearly CH4 balances ranged between 16.2 ± 2.2 and 24.2 ± 5.0 g CH4-C m–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 zero. The net N2O fluxes were low and not significantly different between the four sites. The annual NECB at the rewetted sites was between –183.8 ± 126.9 and 51.6 ± 112.8 g CO2-C m–2 a–1 and at the Sphagnum cultivating site –114.1 ± 48.1 and –75.3 ± 39.8 g CO2-C m–2 a–1. The yearly GWP100 balances ranged from –280.5 ± 465.2 to 644.5 ± 413.6 g CO2-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 g CO2-C m–2 a–1). If the exported carbon 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 after 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 surface. 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.