NITROGEN LEACHING BEFORE AND AFTER CLEAR-FELLING OF FERTILIZED EXPERIMENTAL PLOTS IN A PINUS-SYLVESTRIS STAND IN CENTRAL SWEDEN

Nitrogen leaching before and after clear-felling of experimental plots in a previously nitrogen fertilised Pinus sylvestris stand in central Sweden was estimated by two methods. The main difference between the methods was the estimation of the water flow, which was either simulated by a physically based computer model, SOIL, or calculated from runoff measurements performed by the Swedish Meteorological and Hydrological Institute in the Ljusnan River. Nitrogen leaching was calculated either from the nitrogen concentration in the soil water at 50 cm depth and the simulated vertical water flow, or by multiplying the annual mean nitrogen concentration in the soil water with the annual runoff in the Ljusnan River. Both methods produced similar results. Leaching was estimated from plots treated with ammonium nitrate over a 20 year period at a total rate ranging from 360 to 1800 kg N ha-1, from control plots and from forested reference plots in the adjacent stand. In the year before clear-felling, nitrogen leaching was only enhanced in the 1800 kg N ha-1 treatment, from which the leaching was 3-5 kg N ha-1 compared with about 1 kg N ha-1 from the forested reference and control plots. In the second year after clear-felling leaching was about a third of that before clear-felling, owing to low precipitation and, hence, runoff. Leaching was still only enhanced at the highest fertiliser rate. In the fifth year after clear-felling, leaching tended to increase in all treatments including the control. The additional nitrogen leaching was mainly made up of nitrate. The accumulated nitrogen leaching during the first 5 years following clear-felling was roughly estimated at 2-3 kg N ha-1 for the forested reference plots, the clear-cut control and the 360 kg N ha-1 treatment, 4-5 kg N ha-1 for the 720 and 1080 kg N ha-1 treatments, 14 kg N ha-1 for the 1440 kg N ha-1 treatment and 37 kg N ha-1 for the 1800 kg N ha-1 treatment.