Nutrient limitation in the Biebrza fens and floodplain (Poland)

Fertilization experiments were carried out at four stands of mire vegetation in the Biebrza valley, Poland. The stands included two rich fen communities, a moderate-rich/poor fen vegetation and a floodplain fen. In each stand five replicate sets of treatment plots (50 x 50 cm) were selected randomly. Each set consisted of five plots: a control plot, three plots which were fertilized with N, P, K, respectively and one plot with all three elements together. Fertilizer was applied in April. Groundwater and soil were sampled and analysed twice (April, July). In July aboveground biomass was harvested and nutrient concentrations in plant material were examined. Nutrient release from peat to pore water was measured by incubation experiments. Vascular plants were N-limited in the non-flooded fen communities. The vegetation in the floodplain was not limited by nutrients. The results compare well with the hypotheses of Wassen et al. (1995) on expected nutrient limitation of fens in the Biebrza valley, based on nutrient concentrations in (non-fertilized) mire vegetation. Flowever, the critical nutrient concentration and the P/Nquotient in plant material should be regarded as rough indicators only. We expect N to be the controlling factor for the non-flooded fen communities as long as the fens are not drained and N-input by groundwater inflow and atmospheric deposition remains low.