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Acidification processes in soils, freshwaters, and oceans are natural processes in geological time frames. However, anthropogenic activities on planet Earth have considerably accelerated acidification by enhancing natural processes as well as by changing dynamics, balances, and pathways.

Acidifying substances can have ecosystem external natural sources such as volcanism, dimethyl sulfide (C2H6S) emissions from oceans, or, to a minor extent, sulfide emissions from freshwater wetlands. However, most important are anthropogenic emission sources, mainly fossil fuel combustion processes (e.g., public power plants, industry, and traffic) and agriculture. Emissions of SO2 and NOx to the atmosphere increase the natural acidity of rainwater due to the formation of H2SO4 and HNO3, both being strong acids. Furthermore,

NH 3 emissions mainly from agricultural activity (volatilization from fertilizers and animal liquid manure) trigger acidification processes in soils. After deposition to ecosystems the conversion of NH^ to either amino acids or to NO^ in soils is connected to the production of acidifying H+ ions.

Since the end of the nineteenth century, industrialized regions of the world have been confronted with the consequences of acidic atmospheric deposition, 'acid rain'. There was, and still is, substantial concern about the environmental impacts of air pollution at the local, regional, and global scale. 'Acid rain' has threatened vegetation, wildlife, soil biology, and human health, caused damage to materials, and changed the chemistry of soils and waters.

Anthropogenic land-use changes and use of fossil fuels have further led to dramatically increasing atmospheric CO2 concentrations worldwide. CO2 is absorbed by oceans and reacts with seawater to form H2CO3. Acidification of oceans has adverse effects on marine organisms using CaCO3 in seawater to construct their shells and skeletons (e.g., corals and calcareous phytoplankton).

Acidifying compounds can be carried by winds over long distances and affect ecosystems in pristine areas located hundreds or thousands of kilometers away from pollutant sources. Terrestrial and freshwater ecosystems affected by acidification are usually located in regions where precipitation inputs exceed evapotranspiration, that is, where water percolates through the soil and bedrock.

Acidification is the result of a sensitive (un-)balance between ecosystem internal and external H+ sources and internal H+ sinks of different capacities and reaction rates. Acidification processes in terrestrial and aquatic ecosystems can have natural and/or anthropogenic causes; natural internal or external activities can drive these processes in the ecosystem. These processes and their consequences are discussed in this article.

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