Nitrogen losses and soil water acidity after clear-felling of fertilized experimental plots in a Pinus sylvestris stand

Abstract

Studies of losses of nitrogen to water and air (denitrification) were performed during 3 years following a clearcut of a previously heavily fertilized Pinus sylvestris stand. During a 20-year period prior to the harvest, plots had been fertilized three times with different doses of ammonium nitrate (0–600 kg N ha −1 each time). Soil studies conducted during the last summer before clearcutting showed that the soil nitrogen pool had almost doubled in some plots. Organic nitrogen was the dominant nitrogen fraction in the soil water in most plots. This fraction was independent of the fertilizer dose. Before harvesting and during two subsequent years, nitrate only occurred at the plots which had received the maximum dose (3 × 600 kg N ha −1). The concentrations of the different nitrogen fractions in soil water tended to decrease on all plots from pre-clearcut conditions during the first 2 years. During the third year, which had normal precipitation, increases in nitrate were recorded at the plots with the three highest doses. This was the case especially for the highest dose, for which the concentration rose from 0.24 mg nitrate-N l −1 in the previous year to 2.2 mg l −1. The results as regards the effect of the clearcut on soil water pH were somewhat contradictory, but a minor pH increase was indicated. Before clear-felling there was no correlation between pH of soil water and fertilizer dose. During the years following the clearcut, pH was negatively correlated with dose. The reason for these changes could not be explained solely by nitrification. Losses of N 2O-N were estimated to be less than 0.1 kg ha −1 year −1 during the first 2 years after clearcut. Denitrification measured with the acetylene block technique revealed no higher values, pointing to an insignificant N 2 loss. The losses were independent of the previous fertilizer dose. Despite a huge build-up of the nitrogen storage from previous fertilizations, nitrogen losses to water and air were insignificant during the first 2 years after the harvest. During the third year, which was wetter than the two previous years, increased nitrate concentrations were recorded for the highest dose for the first time. Owing to the cold site and the low precipitation for 2 years after the clearcut, further measurements will be necessary before more definite conclusions can be made.