CO2 exchange of a temperate fen during the conversion from moderately rewetting to flooding

Year-round flooding provides a common land management practice to reestablish the natural carbon dioxide (CO2) sink function of drained peatlands. Here we present eddy covariance measurements of net CO2 exchange from a temperate fen during three consecutive growing seasons (May-October) that span a period of conversion from moderately rewetting to flooding. When we started our measurements in 2009, the hydrological conditions were representative for the preceding 20years with a mean growing season water level (MWGL) of 0cm but considerably lower water levels in summer. Flooding began in 2010 with an MWGL of 36cm above the surface. The fen was a net CO2 sink throughout all growing seasons (2009: -333.312.3, 2010: -294.18.4, 2011: -352.45.1g C m(-2)), but magnitudes of canopy photosynthesis (CP) and ecosystem respiration (Reco) differed distinctively. Rates of CP and Reco were high before flooding, dropped by 46% and 61%, respectively, in 2010, but increased again during the beginning of growing season 2011 until the water level started to rise further due to strong rainfalls during June and July. We assume that flooding decreases not only the CO2 release due to inhibited Reco under anaerobic conditions but also CO2 sequestration rates are constricted due to decreased CP. We conclude that rewetting might act as a disturbance for a plant community that has adapted to drier conditions after decades of drainage. However, if the recent species are still abundant, a rise in CP and autotrophic Reco can be expected after plants have developed plastic response strategies to wetter conditions.