Ökohydrologisches Monitoring von Landnutzungsänderungen

Summary

Due to agricultural intensification during the past decades, the hydrology of peat lands of the northern German lowlands has been altered heavily. Drainage and fertilization resulted in high nutrient losses to aquatic ecosystems and a decline in biodiversity. Within an intensive monitoring programme in Ritzerau, northern Germany, the effects of conversion to organic farming and rewetting on hydrology, nutrient transformations, floristic diversity and the eligibility for extensive cattle grazing are studied in a formerly degraded stream valley wetland. Results indicate that the nutrient transformation and retention performance of this complex is driven by the mutual hydrological exchange rates between the wetland and the stream. Due to the degraded state of the system exchange processes are still short-term. Despite effective short-term retention of up to 99% of NO3-N and 65% of SRP a net export occurred from the stream wetland com-plex. A shifting dominance of wetland and in-stream nutrient retention could be identi-fied. High exports were found for SRP with the falling limb of the hydrograph, when the stream received elevated groundwater discharge from the wetland. The water levels in the wetland were raised by the rewetting measures and grazing is limited. It is shown by the grazing intensity though that during a transition phase, the formerly more inten-sively used part of the wetland is preferred by the cattle despite higher water levels. Plant species numbers have not increased yet, but the less grazed parts of the wet-land show higher abundances of abandonment species, e.g. Phalaris arundinacea. Our ongoing monitoring serves to improve land use and restoration measures for biodiversity and nutrient retention.