Nitrogen saturation in a long-term forest experiment with annual additions of nitrogen

A fertilizer experiment laid out 1971 in a pine (Pinus sylvestris) in N. Sweden is described. The aim was to study the behaviour of a forest ecosystem under controlled nutrient regimes. A primary objective was to study tree production and vitality at different internal nitrogen levels, checked by annual foliar analysis. Tree growth was nitrogen-limited on the site, and the nitrogen regimes ranged from no nitrogen added over near-optimum to excess nitrogen. Interaction with PK addition was studied in a factorial design. Since the question of ecological effects of increasing atmospheric deposition of nitrogen and sulphur was raised by Oden in 1968, this problem became one of the main objectives of this experiment and other experiments in the project The Swedish Optimum Nutrition Experiments in Forest Stands. The experiment started in a young stand which has now been under treatment for more than 20 years. In the beginning nitrogen addition increased tree biomass at all nitrogen levels, but with time the growth increase was reduced at the highest nitrogen regime, partly also at the middle level. Retention of added nitrogen in stand and soil has been high at the lowest level of nitrogen addition, while considerable losses from the sites have occurred at higher levels, indicating nitrogen saturation. Some of the treatment effects on ecosystem properties are described, such as distribution of bioelements between stand and soil and apparent efficiency of needles in stem formation.