| Summary: | The effects of forest clearcutting on the leaching of organic N, NH4+-N and NO3–-N and the processes likely to influence them were studied in two Norway spruce (Picea abies) dominated catchments located on nutrient-rich old peatland drainage areas in southern Finland. The concentrations of NH4+-N in precipitation reaching the ground increased after clearcutting. Those of NO3–-N increased in the area where the atmospheric N deposition was average, but decreased in the area subjected to higher than average N deposition. Ammonium-N concentrations in percolation water passing through the surface peat increased, both under slash-piles and in slash-free areas. Gaseous N2O and N2 emissions or leached amounts of N2O were not influenced by clearcutting. Organic N concentrations in runoff increased immediately after clearcutting, whereas the concentrations of NH4+-N and NO3–-N only increased after the first growing season after cutting. The causes for increased runoff nitrogen concentrations are discussed.
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