NUTRIENT LIMITATION IN THE BIEBRZA FENS AND FLOODPLAIN (POLAND)

Fertilization experiments were carried out at four stands of mire vege tation 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) we re 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 Apri l. 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 w ater was measured by incubation experiments. Vascular plants were N-li mited in the non-flooded fen communities. The vegetation in the floodp lain was not limited by nutrients. The results compare well with the h ypotheses of Wassen et al. (1995) on expected nutrient limitation of f ens in the Biebrza valley, based on nutrient concentrations in (non-fe rtilized) mire vegetation. However, the critical nutrient concentratio n and the P/N-quotient in plant material should be regarded as rough i ndicators only. We expect N to be the controlling factor for the non-f looded fen communities as long as the fens are not drained and N-input by groundwater inflow and atmospheric deposition remains low.