Long-term field experiments simulating increased deposition of sulphur and nitrogen to forest plots

In 1968, experiments with application of sulphuric acid were laid out as a supplement to the project "Optimum nutrition experiments in Swedish forest stands". For a long time the trees appeared little affected by the rather crude treatment but several soil chemical and biological properties were changed by the treatments, both in the field and when simulated in laboratory experiments. The experiments were therefore con tinued and extended, for example by using elementary sulphur as acidifying agent. Independent research groups have found changes in soil biology and vegetation. The lower nitrogen treatments in the optimum nutrition experiments were in the same range as the annual deposition in areas with intensive agriculture in middle Europe. The observed changes in soil chemistry and biology, as well as in tree growth and vegetation, are briefly described. It is concluded that several changes, including a case of short-term increase in tree growth, might be ascribed to shock effects of the sulphuric acid applications. On the other hand, reduced soil respiration, persisting up to 11 yr after the last acid application, is an example of a chronic effect. In many cases elementary sulphur applications had effects similar to those of sulphuric acid, but weaker, and shock effects were less likely here. In all cases nitrogen applications increased tree growth, but the larger amounts (60-90 kg N ha-1 yr1) became supraopti mum with time. Nitrogen compounds (ammonium nitrate and urea) increased pH in the humus layer, but acidified the mineral soil, especially at higher rates of application. As the soil and vegetation effects of nitrogen application operate in the same direction as those observed after natural disturbances of the ecosystem, it is concluded that possible "shock" effects fall within a range naturally occurring in boreal forests. Two results of particular interest: i) factorial combinations of acid and nitrogen compounds were more effective acidifiers of the mineral soil than either agent alone, ii) much of the soil acidification can be explained by the "mobile anion" concept, and that the soil is depleted of, i.a., magnesium and calcium, which are replaced to a large extent by aluminium.