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Abstract

In peatland forestry, fertilization is often needed to reach a good yield. Phosphorus and potassium are mainly used, but on nutrient poor fens and bogs nitrogen also has to be added. These fertilizers affect the environment and thereby influence the runoff waters. This essay concerns the first three months after fertilization, of which the first two weeks have been paid particular attention. As the mires of the sub-basins were sedge fens, partly poor fens, with a pine stand in some areas but mostly treeless, the fertihzers used were ammonium nitrate (100 kg N) rock phosphate (50 kg P) and potassium chloride (100 kg K). The fertilization was performed from the air. During the very first hours after fertilization, drastic changes in water chemistry were found. In one area pH dropped 0.3 units while at the other no immediate change was seen. For the whole three months of the investigation period the decreases in pH were in the range 0.1-0.5 units. Nitrogen concentration reached a peak of 260 mg/l, phosphorus 5 mg/l and potassium about 60 mg/l. These high values were of short duration but the concentrations were considerably increased during one week. Later, partly due to decreasing discharge, the water chemistry became almost similar to that measured under unfertilized conditions. The main loss of fertilizer occurred during th e first two weeks and amounted to 22% of the applied nitrogen, about 1% of the phosphorus and 5-9% of the potassium. High concentrations of ammonium and nitrate may occur after fertilization and together with drops in pH may be hazardous to fish. Increased concentrations of aluminium dissolved from stream bottoms due to decreased pH may also be toxic to fish. In this investigation no fish kill or changes in fish populations were found. In runoff waters with pH around 7 the high ammonium concentrations found in the Siksjöbacken study could cause lethal or sublethal effects on fish due to increased concentrations of ammonia (NH3).