Global Element Cycles

Most element cycles are not closed entitles In ecosystems. This Is particularly true for water, carbon and the nutrients nitrogen and sulphur. At the scale of landscapes resources are often imported and exported from one ecosystem to another. The figure shows the export of soluble organic carbon in humic and fulvic acids, which co-transport heavy metals, from boreal pine forests of Siberia. The dissolved organic carbon reaches the ocean via streams and rivers. The river is the Dubces, a tributary of the Yenisei. Photo E.-D. Schulze

Element cycles (Fig. 5.2.1; Schulze 2000) of water, carbon, nitrogen and sulphur are characterised (1) by the degree of storage in the atmosphere, sea and on continents; (2) by the turnover between these compartments, the so-called fluxes; and (3) the turnover within the compartments, the so-called internal cycles of ecosystems. The turnover between the earth's surface and the atmosphere is essentially controlled by organisms, whilst the turnover in the atmosphere is predominantly dependent on energy from solar radiation. Global fluxes between continents, oceans and the atmosphere will be considered later. For comparison, it is important to know that the surface of the earth is about 505 million km2 of which about 353 million km2 are oceans, i.e. 70% of the earth's surface is covered by oceans. The area of continents is about 152 million km2, but only 135 million km2 (88% of continents) are ice-free. Europe (the 15 countries of the European Union, EU15) has an area of 3.2 million km2.

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