When soil has a high content of easily weathered minerals it can absorb a relatively large amount of acid deposition without becoming acidic. But if the minerals in the soil do not weather easily, there is little natural resistance.
Soil is acidified slowly as a result of natural processes. This has been going on since the end of the last ice age, but has been greatly accelerated by forestry and acid deposition. The most serious consequences of soil acidificaiton are the following:
-Plant nutrients, particularly base cations (mainly magnesium, potassium and calcium), are leached out by the added acid;
-Toxic metals are freed;
-Vital phosphates become bound and therefor less accessible to plants.
Acidification of surface water
The natural buffering system in lakes and waterways is provided by bicarbonate (HCO3 -), that reaches the water from the surrounding land. Bicarbonate is released by the weathering of minerals on land and during the decomposition of organic matter. Lakes and waterways that are surrounded by easily weathered types of soil or cultivated land are constantly fed with significant amounts of bicarbonate and therefore generally have good resistance to acidification. However, water that is surrounded by soil that does not weather easily usually has limited buffering capacity, and acidification can occur if acid is added.
The limits to what “nature can tolerate" are called critical loads. The international definition of critical load for acidification is “the highest deposition of acidifying compounds that will
not cause chemical changes leading to long-term harmful effects on ecosystem structure and function". Although the definition leaves some room for interpretation, its foundation is that the limits must be set for the most sensitive organisms and ecosystems.
The most dominant cause of today´s problems is the airborne deposition of acidifying substances — sulphur, nitrogen oxides and ammonia — as well as the harvesting of biomass.
Soil and surface water are gradually able to recover their original chemical identity; however, it is not certain that the ecosystems will be restored. The extent to which this happens depends, among other factors, on the ability of various species to spread. And some things will never be the same. For example the impoverishment of genetic diversity within species that has been caused by acidification is irreversible.
Source: Air and the environment by P.Elvingson and C.Ågren
Guidance on GAINS application (pdf, 2.4 MB) model in the state environmental management system of the Russian Federation (Russian; English version is under development)
Guidance document on Critical Load assessments (in Russian) (pdf, 790.3 kB)