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Critical loads

The term ”Critical Load” is used in connection with the international goal of limiting excessive air pollution.

Critical load is defined as "The load of one or more pollutants where significant harmful effects on selected sensitive elements of nature and the environment will not occur, assessed using current knowledge". Critical loads contain a political element, as it should be determined what a significant impact is and which elements of nature and the environment are to be protected. However, determining the load is based on scientific methods, which for heaths tare primarily based on results from plant competition models. These model calculations do not fully take into account the effect of heavy soil acidification.

The term indicates that each habitat has a maximum limit to how much of a given type of atmospheric deposition it can withstand each year. This amount corresponds to the critical load. The critical load depends on several factors that are dependent on location, e.g. soil, rainfall and species composition.

On a local scale, the critical load for nitrogen can be used to reduce emissions of nitrogen from livestock. This will ensure that vulnerable nature is protected. On a larger geographical scale, critical load scan be used for negotiations on reduction of State discharge of various types of air pollution.

The currently valid published critical load for the habitat wet heathland with bell heather is between 10–25 kg N ha-1year-1. It is a so-called empirical critical load, which means that it is determined based on a number of studies on heaths with bell heather. The fact that there is a range partly means that the critical load is dependent on local conditions, partly that regular maintenance, removing nitrogen from the ecosystem, theoretically enables it to tolerate more deposition without suffering eutrophication.

See also: Strandberg og Mortensen, 1996.

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