Response of two coastal peatlands to the duration of rewetting and the release of nutrients

Along the German coastline of the Baltic Sea are numerous former coastal fens. The originally approx. 40,000 hectares of peatland that were regularly flooded are now mostly diked, drained and are mainly used for agriculture. In order to be prepared for rising sea levels in the future, attempts are being made to determine the consequences of renewed flooding for the development of the fen and the adjacent Baltic Sea. A first test area was a former peatland in Drammendorf on the island of Ruegen, Germany. It had been rewetted in a major campaign lead by the Baltic Sea Foundation. The other area, Karrendorf, had been flooded already 30 years ago. The two coastal fens had different periods of waterlogging and were studied comparatively over the course of a year. In addition to the typical seasonality, which is reflected in the same temperature and oxygen content at both sites and the adjacent bays, there are striking differences in nutrient concentrations and isotope signatures of organic matter and nitrate, some of which can be traced back to the history of the areas as agricultural land. Shortly after its rewetting in winter, for example, nitrate concentrations in Drammendorf were several times higher than in Karrendorf and nitrous oxide saturations were up to 4000 %.

We found that the longer the area was flooded and nutrients released to the overlying waters, the less nutrients are discharged into the adjacent Baltic Sea. This finding was not only reflected by lower nutrient concentrations in the surface water of Karrendorf, but also in much lower porewater nutrient concentrations. Moreover, we saw a close coupling of the microbial processes in the overlying water with the soils. Nitrification, which converts ammonium from decomposition processes into nitrate, was clearly recognizable in the nitrogen and oxygen isotopes of nitrate despite low rates. However, high δ¹⁸O values of nitrate indicate precipitation as another possible source of nutrients. In addition, the isotope values of the organic matter show that there is an intensive exchange between the Baltic Sea and the coastal fens, whereby Drammendorf is more strongly characterized by marine organic matter inflow than Karrendorf. Thus, the flooding of coastal peatlands initiates lateral transport across the terrestrial-marine interface, while the microbial processes play more of a role in the interaction between soil and water.

For future management, the soil composition and history should always be considered and the intensity of exchange with coastal waters taken into account so that these areas do not become hot spots of eutrophication due to their large nutrient reservoirs.