The great bulk of living matter in any community is water. The rest is made up mainly of carbon compounds (95% or more) and this is the form in which energy is accumulated and stored. The energy is ultimately dissipated when the carbon compounds are oxidized to carbon dioxide (CO2) by the metabolism of living tissue or of its decomposers. Although we consider the fluxes of energy and carbon in different chapters, the two are intimately bound together in all biological systems.
Carbon enters the trophic structure of a community when a simple molecule, CO2, is taken up in photosynthesis. If it becomes incorporated in net primary productivity, it is available for consumption as part of a molecule of sugar, fat, protein or, very often, cellulose. It follows exactly the same route as energy, being successively consumed, defecated, assimilated and perhaps incorporated into secondary productivity somewhere within one of the trophic compartments. When the high-energy molecule in which the carbon is resident is finally used to provide energy for work, the energy is dissipated as heat (as we have discussed in Chapter 17) and the carbon is released again to the atmosphere as CO2. Here, the tight link between energy and carbon ends.
Once energy is transformed into heat, it can no longer be used by living organisms to do work or to fuel the synthesis of biomass. (Its only possible role is momentary, in helping to maintain a high body temperature.) The heat is eventually lost to the atmosphere and can never be recycled. In contrast, the carbon in CO2 can be used again in photosynthesis. Carbon, and all other nutrient elements (e.g. nitrogen, phosphorus, etc.) are available to plants as simple inorganic molecules or ions in the atmosphere (CO2), or as dissolved ions in water (nitrate, phosphate, potassium, etc.). Each can be incorporated into complex organic carbon compounds in biomass. Ultimately, however, when the carbon compounds are metabolized to CO2, the mineral nutrients are released again in simple inorganic form. Another plant may then absorb them, and so an individual atom of a nutrient element may pass repeatedly through one food chain after another. The relationship between energy flow and nutrient cycling is illustrated in Figure 18.1.
By its very nature, then, each joule of energy can be used only once, whereas chemical nutrients, the building blocks of biomass, simply change the form of molecule of which they are part (e.g. nitrate-N to protein-N to nitrate-N). They can be used again, and repeatedly recycled. Unlike the energy of solar radiation, nutrients are not in unalterable supply, and the process of locking some up into living biomass reduces the supply remaining to the rest of the community. If plants, and their consumers, were not eventually decomposed, the supply of nutrients would become exhausted and life on the planet would cease. The activity of heterotrophic organisms is crucial in bringing about nutrient energy cannot be cycled and reused; matter can ...
cycling and maintaining productivity. Figure 18.1 shows the release of nutrients in their simple inorganic form as occurring only from the decomposer system. In fact, some is also released from the grazer system. However, the decomposer system plays a role of overwhelming importance in nutrient cycling.
The picture described in Figure 18.1 is an oversimplification in one important respect. Not all nutrients released during decomposition are necessarily taken up again by plants. Nutrient recycling is never perfect and some nutrients are exported from land by runoff into streams (ultimately to the ocean) and others, such as nitrogen and sulfur, that have gaseous phases, can be lost to the atmosphere. Moreover, a community receives additional supplies of nutrients that do not depend directly on inputs from recently decomposed matter - minerals dissolved in rain, for example, or derived from weathered rock.
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