Waldkalkung - Bodenschutz contra Naturschutz?

The deposition of strong inorganic acids in forests leads to low pH values and high concentrations of Al3+ in the soil. Forest liming neutralizes acidity and leads to an increase in base cations. Additionally, surface organic layers decay more rapidly and stored nutrients are mobilized. The ground vegetation responds to liming with an increase in nutrient-loving species and a decline in acidophytes. The mycorrhizal fungi and soil fauna species composition change dramatically. Tree roots, at least initially, retreat from the topsoil. It takes several years for the base cations to reach the deeper soil horizons (up to about 30 cm) and they rarely go deeper. Since 1990 sulfur deposition has greatly declined. However N deposition rates remain at high levels (conifer stands > deciduous > open areas). High N inputs continue to contribute to soil acidification and at the same time cause the eutrophication of forest ecosystems which are naturally N-limited. The increased rate in forest growth results in an increased need for other nutrients. In many forests, the critical deposition threshold (“critical load”) of approximately 10 to 20 kg N ha -1 yr -1 has been exceeded. In stands where the soils are supersaturated with nitrogen, the nitrate which is not retained by the humus or assimilated by the forest vegetation leaches into the ground water. Up until today in Baden-Wuerttemberg, Rhineland-Palatinate, Hesse, Lower Saxony, North Rhine-Westphalia, Thuringia and recently in Saxony-Anhalt large areas of forest have been limed using three to four tons of dolomitic lime per hectare per decade. The aim is to prevent the ongoing acidinduced degradation of clay minerals in order to increase forest vitality. Often P and / or K are added to the lime. Bavaria, Brandenburg and Mecklenburg-Western Pomerania either do not lime their forests or only do so in a very restricted manner. In these states the central European tree species beech, spruce, Scots pine, fir and oak growing there are no less vital than elsewhere. These tree species grow in a wide range of ecological soil types. Analyses of leaves and needles show that a small, but adequate nutrient supply exists even in the most acidic forest soils. Today it is not soil acidification but eutrophication (and climate change) which are the main threats to forest ecosystems, especially for oligotrophic communities on sandy soils. Liming in eutrophic forests counteracts acidification and eventually leads to deeper rooting. But it also increases nitrogen availability and other limiting nutrients and adds to the process of eutrophication. Therefore, the results evaluating of the effects of liming are ambivalent. The problem can only be solved by a reduction in N deposition. From a nature conservation standpoint it is of particular concern that liming is done on oligotrophic forest communities which are naturally low in base substrates. These habitats need to be protected from the addition of lime. In mesotrophic clay soils susceptible to acidification, liming can be occasionally tolerated. The enrichment with P and K means fertilisation and is therefore not acceptable. In order to weigh the impacts of forest liming, unlimed control plots should be established. In the light of today’s relatively high forest growth rates further funding for forest liming should be reassessed