Nitrogen, Phosphorus and Potassium Budgets for Two Small Fens Surrounded by Heavily Fertilized Pastures

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Abstract
In the Vechtplassen area of The Netherlands, small fens are scattered in an agricultural landscape which is used for dairy production. Fens in groundwater discharge areas have been less affected by eutrophication than those in groundwater recharge areas. Hydrological and nutrient budgets (nitrogen, phosphorus, potassium) were measured in two fens from 1985 to 1987. One fen, located in a groundwater recharge area, received polluted river water which compensated for evaporative and groundwater losses during the sumemr. This inflow, and precipitation, were the major inputs of water. Major hydrological outputs were evapotranspiration (summer) and subsurface outflow (winter). The second fen was located in a groundwater discharge area. It received river water only occasionally and the main hydrological inputs were groundwater discharge and precipitation. Evapotranspiration and subsurface outflow were the major hydrological outputs. Although nutrient inputs, especially nitrogen, were enhanced as a result of agricultural activities, summer harvesting of above-ground vegetation resulted in a net loss of potassium and phosphorus from the fens. Nitrogen inputs almost equalled outputs. Differences between the fens in their overall nutrient budgets were small. Annual differences in production of biomass and in nutrient uptake by the vegetation could not be explained by differences in the nutrient budgets. Primary production and nutrient uptake reflected differences in mineralization rates of nitrogen and phosphorus. The mineralization process is regulated by the amount of organic nutrient substrates which may be mineralized as well as by the substrate composition and the ionic composition of the fen water. The young fens are primarily fed by external nutrient inputs and are characterized by nutrient accumulation in peat. In mature fens, the peat which has developed isolates the fen from its environs, and plant growth is driven by re-mineralization of nutrients that have accumulated in former periods. Harvesting of vegetation effectively reduces phosphorus and potassium in the soil. Over the long term, a shift from nitrogen limitation towards phosphorus or potassium limitation is to be expected in the fens. However, regional land-use control is necessary to prevent a further increase in nutrient inputs.