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Last changed: 4/04/11
Inputs of sulphur and nitrogen compounds cause more acid soil conditions (lowered pH value) and a loss of nutrients. As a result, the amounts and composition of nutrients available to vegetation is changed. Long periods of acid stress lead to an imbalanced nutrient uptake and reduced vitality of plants (e.g. trees), and to limited resistance to natural stress factors. Many ecosystem functions are then limited, especially water filtration. Thus, forest damage is in fact damage not to trees only but to forest ecosystems.
Furthermore, pollutant deposition lead to wide-ranging degradation of soil conditions to unfavourable acidified levels. Since plants and plants communities that only thrive in neutral soil conditions are displaced by competing species and communities that do thrive in acidic environments (e.g. wavy hairgrass), acid inputs lead to a decrease in the diversity of vegetation. As many animal species are entirely dependent on certain plant species, the continuous decline in plant species diversity which has been observed in Germany also affects the diversity of animal species. Fauna must also bear the immediate consequences of acidification: for instance, earthworms that inhabit mineral soils can not exist in acidified soil (lower than pH 4), as aluminium, which has a toxic effect on them, is dissolved in this range. However, there is no ”ecological substitute” for earthworms. This is another example of geochemical changes having ecological and functional effects.
Inputs of acidifying sulphur compounds have declined in recent years, although no such positive trend can be detected for acidifying nitrogen inputs. The critical loads for sulphur inputs continue to be exceeded on 85% of the surface of sensitive ecosystems in Germany: these areas continue to acidify. Recovery of ecosystems to pre-industrial era levels can, depending on soil and ecosystem, take several hundred years’ time even if critical loads are not exceeded.
Up until the mid-1990s inputs of acidifying substances were highest in the forest areas of the Central German Uplands. High deposition rates of sulphur compound as a consequence of industrial combustion processes were a main cause, particularly in the 1970s and 1980s, of massive exceedance of critical loads.
These exceedances have been reduced thanks to the success achieved by international treaties to reduce transboundary air pollution. In the Laender of former West Germany, clean air policy measures such as the Federal Immission Control Act (BImSchG) or the Technical Instructions on Air Quality (TALuft) led to significant abatement of inputs as of the mid-1980s. It was only after 1990 that this process was emulated in the new Laender of reunified Germany.
Nevertheless, the extensive inputs of the past have long-term effects: the soils in areas with previously high levels of sulphur deposition will remain acidified for a long time. Lime has been spread at many sites to compensate for acid deposition.
The excessive input of nitrogen compounds (mainly (H)NO3 and ammonium (NH4+) from nitrogen oxides (transport) and ammonia from agricultural sources, animal husbandry in particular), leads to soil acidification. Constant levels of nitrogen inputs from the atmosphere during the last 15 years have turned nitrogen into the most significant acidifier and therefore main cause of continued acidification of semi-natural ecosystems. The nitrogen input is converted to nitrate (NO3-) in the soil, which can be leached from the soil together with nutrients of basic reaction (magnesium, calcium, potassium). This results in imbalanced nutrient composition of soils and plants, elevated nitrate concentrations in soil leachate and groundwater, as well as widespread leveling of soil conditions to adverse acidified levels.