The influence of rewetting on vegetation development and decomposition in a degraded fen

The rewetting technique border irrigation was installed in a degraded fen peatland in northeastern Germany. Because of the prevailing site conditions, the technique resulted in two different rewetting variants (surface irrigation and temporary inundation) at the study site. This paper reports on the practicability of this technique and the influence of rewetting on vegetation development, decomposition processes and soil nutrient availability, and the possibilities for renewed peat accumulation. The technique proved to be suited for rewetting fen sites with a continuous slope, deep peat layer with low hydraulic conductivity, and upstream water recharge facilities. A subsidence of the ground-water levels during the summer months, however, could not be avoided in dry years. The vegetation changed slowly from species-poor grassland into typical fen plant communities, despite rewetting and soil tillage. Species richness, however, was higher in the surface irrigation than in the temporary inundation variant. A sufficient water supply proved to be absolutely necessary to retard decomposition processes because higher decomposition of root materials (i.e., higher k values) occurred under temporary inundated conditions. Generally, the higher water content in the soil after rewetting led to a lower nitrate-N-to-ammonium-N ratio in the topsoil in both rewetting variants. In the surface irrigation variant the mineral nitrogen content (Nmin) of the topsoil decreased from 7.8 to 4.4 g N/m(2), which is also correlated with the increase in water content of the soil. The low Nmin levels of fens which were never deeply drained (0.9-2.8 g N/m(2)), however, were not reached within the observation period of 3 years.