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Stores and Pathways
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Biological or nutrient cycling refers to the circulation of chemical elements from environment to organisms and back again to the environment. Figure 4.9 illustrates a simple model of nutrient circulation. Nutrient elements are stored in three compartments within the ecosystem: in the soil, in the living biomass and in surface litter. Nutrients are recycled between environment and organisms via three main pathways.
(1) In the ‘uptake’ or ‘growth’ pathway, simple inorganic elements and compounds of nitrogen, phosphorus and potassium, together with those of other essential elements, are taken up from the soil and converted into complex, organic substances within the living biomass. During this growth phase other elements, e.g. carbon and oxygen from the atmosphere and hydrogen in water, are also incorporated.
(2) As plants and animals die, they contribute nutrients to the litter store. This is the ‘fallout’ pathway.
(3) A ‘decay’ pathway is formed by the decomposition of litter to humus, with the eventual release of inorganic nutrients back into the soil.
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The storage of nutrients within an ecosystem is not a static condition. Nutrients can increase or decrease, often rapidly, over a period of time. For this reason the three-compartment/pathway model in Figure 4.9 is not self-contained or ‘closed’. Other pathways exist, which add nutrients to or remove nutrients from the system. For instance, chemical elements are increased within the soil compartment by rock-weathering. They may also enter the ecosystem from other ecosystems in rainfall (precipitation) and as wind-blown dust and leaves. Animals moving from one ecosystem to another may also alter the respective balance of nutrients. On the other hand, chemical elements can be lost from one ecosystem to adjoining ecosystems mainly by run off (erosion) and the leaching of soil.
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Humans and nutrient cycing
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Humans can alter fairly easily the composition, storage and circulation of chemical elements within ecosystems. An increase in the storage of nutrients occurs when inorganic fertilizers, rich in nitrogen, phosphorus and potassium, are added to soils. Provided that the amounts are not excessive, such fertilizers will increase crop yields and thus speed up the rate of nutrient cycling. A good example of an indirect exchange of chemical elements from one ecosystem to another is sulfur from coal-burning power stations in Britain being carried by winds to Scandinavia. Many habitats there, including lakes and soils, are being made more and more acid by excessive sulfur deposition, mostly in the form of acid rain or weak sulfuric acid (H2SO4). This has resulted in a decrease in fish and a decrease in the growth of crops and forests, and thus in a decline in the rate of nutrient cycling.
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Human activity may reduce the nutrient resources of an ecosystem by misusing and over-exploiting the land. Figure 4.10 illustrates one such example where nutrients are extracted at a rate faster than they are replaced. In the Third World, two-thirds of the population relies on wood for cooking and heating, but almost half of these people face shortages of fuel wood. To meet their needs, they are forced to burn firstly the crop stalks, which remain after the harvest, and then, when the crop stalks have run out, animal dung. Normally the crop stalks would be fed to the animals, whose dung could be used as a fertilizer for the next crop. This would recycle some of the nutrients taken out of the soil in crop (biomass) harvest. But the shortage of fuel wood means that the fertility of the topsoil is not being replaced. Once deprived of the nutrients and the organic remains needed for structural stability, topsoil erodes more easily.
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