The sources of all elemental nutrients necessary for life are abiotic pools, the atmosphere, oceans, and sediments. The atmosphere is the primary source of nitrogen, carbon (as carbon dioxide), and water for terrestrial ecosystems. Sediments are a major pool of carbon (as calcium carbonate), as well as the primary source of mineral elements (e.g., phosphorus; sulfur; and cations such as sodium, potassium, calcium, and magnesium released through chemical weathering). The ocean is the primary source of water, but it also is a major source of carbon (from carbonates) for marine organisms and of cations that enter the atmosphere when winds >20 kph lift water and dissolved minerals from the ocean surface.
Resources from abiotic pools are not available to all organisms but must be transformed (fixed) into biologically useful compounds by autotrophic organisms. Photosynthetic plants acquire water and atmospheric or dissolved carbon dioxide to synthesize carbohydrates, which then are stored in biomass. Nitrogen-fixing bacteria and cyanobacteria acquire atmospheric or dissolved N2 and convert it into ammonia, which they and some plants can incorporate directly into amino acids and nucleic acids. Nitrifying bacteria oxidize ammonia into nitrite and nitrate, the form of nitrogen available to most plants. These autotrophs also acquire other essential nutrients in dissolved form. The living and dead biomass of these organisms represents the pool of energy and nutrients available to heterotrophs.
The size of biotic pools represents storage capacity that buffers the organisms representing these pools against reduced availability of nutrients from abiotic sources. Larger organisms have a greater capacity to store energy and nutrients for use during periods of limited resource availability than do smaller organisms. Many plants can mobilize stored nutrients from tubers, rhizomes, or woody tissues to maintain metabolic activity during unfavorable periods. Similarly, larger animals can store more energy, such as in the fat body of insects, and can retrieve nutrients from muscle or other tissues during periods of inadequate resource acquisition. Detritus represents a major pool of organic compounds. The nutrients from detritus become available to organisms through decomposition. Ecosystems with greater nutrient storage in living or dead biomass tend to be more resistant to certain environmental changes than are ecosystems with more limited storage capacity (Webster et al. 1975).
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