Nutrient supply in undrained and drained Calthion meadows

. Plant species‐rich Calthion meadows on mesotrophic fen peat soil extensively cut for hay are among the endangered semi‐natural vegetation types in northwestern Europe. They are often badly affected by lowering the groundwater table (drainage) and fertilization. In a comparative study of an undrained site with a Calthion meadow and an adjacent drained site, availability of N, P and K was biologically assessed under field conditions (for two years) as well as in a greenhouse (for 18 weeks) by measuring shoot responsiveness. Also, experimental wetting of intact turf samples taken from both sites was applied in order to study the interaction between nutrient supply and anaerobic soil conditions. It was concluded that the above‐ground phytomass yield in the undrained site was restricted by a major shortage of N‐supply and a moderate shortage of K‐supply by the fen peat soil. The above‐ground phytomass yield of the drained site was only reduced by a strongly limited supply of K by the soil. The extent of K‐deficiency was larger for the drained site. No P‐deficiency was observed in any of the drained or undrained sites. Rewetting turf samples, taken from the drained site, did not change above‐ground phytomass yields, suggesting that nutrient supplies were not affected by rewetting. Leaching has likely resulted in a strong reduction of K‐supply in the drained site. It is assumed that a shortage in K‐supply from the peat soil may have become an important environmental constraint for characteristic plant species of Calthion meadows. This may hamper the development of this meadow type on drained peat soils after rewetting by groundwater discharge.