Increased leaching of nitrate at two forested catchments in Finland over a period of 25 years

Abstract

Relationships between runoff and water quality from the 1960s to the present time were studied in two adjacent, almost undisturbed forested catchments (Teeressuonoja, 0.69 km 2, and Yli-Knuutila, 0.07 km 2) in southern Finland. The concentrations of nitrate-nitrogen (NO 3N) in stream-water have clearly increased in both the catchments during the last 20–25 years. In Yli-Knuutila, the volume-weighted average concentration of NO 3N increased over twofold, from 580 μgl −1 in 1969–1979 to 1350 μgl −1 in 1980–1990. In Teeressuonoja, the nitrate levels were lower but the increasing trend was comparable with that of Yli-Knuutila. The largest increase of concentrations of NO 3N occurred from the mid-1970s to the mid-1980s. The total nitrogen stream-water output of Yli-Knuutila increased almost threefold, from 1.1 kg N ha −1 year −1 in 1969–1979 to 2.9 kg N ha −1 year −1 in 1980–1990. The average proportion of the NO 3N output of the total nitrogen output increased during the same period from 56% to 68%. The mean annual inorganic deposition of nitrogen increased from 6.2 kg ha −1 year −1 in 1971–1979 to 8.5 kg ha −1 year −1 in 1980–1990. Decreasing retention was reflected in the input-output budget of inorganic N at Yli-Knuutila; output divided by input increased from 12% during the 1970s to 24% during the 1980s. Increased leaching in the Yli-Knuutila and Teeressuonoja catchment streams indicates the effects of increased nitrogen deposition. The increase in nitrate leaching in Yli-Knuutila, particularly during the summer months, provides evidence of nitrogen saturation and the inability of the biomass to utilize the N available, whereas increase during snowmelt and the autumn period indicates the impact of N deposition.