Comparison of nitrogen and phosphorus fluxes in some European fens and floodplains

Abstract

Question: How do nitrogen and phosphorus budgets and balances differ between eutrophic fens and floodplains in western Europe and fens and floodplains in Poland, where we expect less eutrophication to occur? Location: Wetlands along the rivers Dommel (The Netherlands), Zwarte Beek (Belgium) and Biebrza (NE Poland). Methods: Assessment of external input and output fluxes as well as net N-mineralization rates. Annual N- and P-balances were estimated by the sum of all external input and output fluxes: atmospheric deposition, input of dissolved matter by flooding, input of sediment by flooding, input by groundwater, output by leaching, output by hay-making and for N also input by N2-fixation. For N we also estimated net annual N-availability for plant growth, i.e. the N-budget, which includes net mineralization in soil. Results: The studied wetland sites had a negative balance, which means that nutrients are depleted but only if mown annually, except for the Dutch/Belgian fens which had an equilibrium N-balance and the Polish fen which had an equilibrium P-balance. For the N-budget it appeared that atmospheric deposition added significantly to the budget of Dutch/ Belgian fens and N-mineralization added significantly to fen and floodplain budgets, except for the Polish fens. Mineralization dominates the N-budget of the western European floodplains. Hay-making is the most important output pathway, particularly if practised annually. It seems to diminish Nenrichment in the Dutch fens and floodplains. Conclusions: We conclude that western European fens and floodplains as well as Polish floodplains have a significant positive N-budget indicating that there is a surplus of N for plant growth. In the Polish fens this is less due to low atmospheric deposition and lower N-mineralization rates. The latter is associated with less drying out of the studied Polish ecosystems in summer. Our approach, although an approximate quantification, is helpful for assessing priorities focused on nutrient management.