The study of biogeochemical cycles in ecosystems requires not only extensive observations (see introductory page to Chap. 3), but also experiments outside the laboratory (see Mooney et al. 1991). Technical requirements for such sites in the field are demanding. An example is the "Shelter" experiment in a forested watershed conducted by the EU project "Exman" The canopy was separated from the roots by a roof. Rainwater was collected from this cover and pollutants (sulfur and nitrogen deposition) were removed. The purified rainwater was then returned to the soil. Thus it was possible to separate the effects of pollutants on the crown of the trees from the effects of acid deposition on the roots. As it is not possible to derive statistically significant information from one plot, the experiment was repeated at five sites within the EU. Gorsjon, southern Sweden. Photos E.-D. Schulze
The interactions between autotrophic and heterotrophic organisms of an ecosystem have been combined in the biogeochemical cycles, where the coupling between the trophic levels only occurs by making substrates available. In each ecosystem many alternative paths exist for the synthesis and decomposition of materials. Physico-chemical conditions and composition of species in the ecosystem often decide which path is used preferentially within the nutrient network. Feedback effects of organisms on the physicochem-ical boundary conditions occur mainly via weathering and loss of minerals as well as the heat and water balances (for change of optical characteristics and surface roughness, see Chaps. 2.1 and 2.2).
According to the extent of exchange of substances with the environment, i.e. material leaves the ecosystem or is re-cycled into the ecosystem, a distinction is made between open and closed cycles. Transitions are possible and, indeed, under natural conditions, transitions are the rule rather than the exception. Open cycles occur
• with water: the requirement for water is so large that storage is not possible;
• with carbon: as plants obtain C02 from the free atmosphere and C02 is released back to the atmosphere through the respiration of autotrophic and heterotrophic organisms; transfer of organic carbon from heterotrophic to autotrophic species can be neglected (amino acids);
• in managed systems (grazing, agriculture and forestry): Wherever material is taken out of the ecosystem and perhaps other materials (fertilisation) are added, the biogeochemical cycles are open;
• in ecosystems subjected to high input of S and N from the atmosphere; intermediate storage in the system may occur (e.g. for N) which, in the first instance, accelerates turnover within the ecosystem. Release of these elements to the atmosphere or to groundwater may occur later or be started earlier by external disturbances.
Closed cycles are found under favourable conditions
• for nitrogen, e.g. when the external supply of N and denitrification are low compared with the internal turnover in the ecosystem;
• for cations and phosphate, although losses of these elements through leaching cannot be excluded. The balance between weathering of primary minerals and leaching determines the succession of the ecosystem.
The ideal view of closed material cycles has not been achieved, even in ecosystems undisturbed by man (Chadwick et al. 1999; see Fig. 3.5.1). In the following, the turnover of water and the carbon and nitrogen cycles are discussed in more detail.
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