Living organisms expend energy to extract chemicals from their environment, hold on to them and use them for a period, and then lose them again. In this chapter, we consider the ways in which the biota on an area of land, or within a volume of water, accumulates, transforms and moves matter between the various living and abiotic components of the ecosystem. Some abiotic compartments occur in the atmosphere (carbon in carbon dioxide, nitrogen as gaseous nitrogen), some in the rocks of the lithosphere (calcium, potassium) and others in the hydrosphere - the water of soils, streams, lakes or oceans (nitrogen in dissolved nitrate, phosphorus in phosphate).
Nutrient elements are available to plants as simple inorganic molecules or ions and can be incorporated into complex organic carbon compounds in biomass. Ultimately, however, when the carbon compounds are metabolized to carbon dioxide, 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. By its very nature, each joule of energy in a high-energy
Figure 18.23 Annual variations in the atmospheric increase in carbon dioxide (circles and black line) and in carbon released (histograms above the midline) or accumulated (histograms below the midline) in the global carbon cycle from 1980 to 1995. (After Houghton, 2000.)
Land use Q Oceanic uptake
Fossil fuels -»-Atmospheric increase
Residual terrestrial sink
1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995
Year compound can be used only once, whereas chemical nutrients can be used again, and repeatedly recycled (although nutrient cycling is never perfect).
We discuss the ways that nutrients are gained and lost in ecosystems and note that inputs and outputs of a given nutrient may be in balance. However, this is by no means always so, in which case the ecosystem is a net source or sink for the nutrient in question. We discuss the components of nutrient budgets, and the factors affecting inputs and outputs, in forests, streams, lakes, estuaries and oceans.
Because nutrients are moved over vast distances by winds in the atmosphere and by the moving waters of streams and ocean currents, we conclude the chapter by examining global biogeo-chemical cycles. The principal source of water in the hydrological cycle is the oceans; radiant energy makes water evaporate into the atmosphere, winds distribute it over the surface of the globe, and precipitation brings it down to earth. Phosphorus derives mainly from the weathering of rocks (lithosphere); its cycle may be described as sedimentary because of the general tendency for mineral phosphorus to be carried from the land inexorably to the oceans where ultimately it becomes incorporated in sediments. The sulfur cycle has an atmospheric phase and a lithospheric phase of similar magnitude. In contrast, the atmospheric phase is predominant in both the carbon and nitrogen cycles. Photosynthesis and respiration are the two opposing processes that drive the global carbon cycle while nitrogen fixation and denitri-fication by microbial organisms are of particular importance in the nitrogen cycle. Human activities contribute significant inputs of nutrients to ecosystems and disrupt local and global biogeochemical cycles.
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