Primary Saprotrophs Detritivores Roots

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SOIL SOLUTION (dissolved organic matter and inorganic ions)

Fig. 6.2. Litter transformations and uptake through decomposition. Roots are the main sink of soil solution molecules into primary production. Metabolic wastes and excreted organic matter from saprotrophs contribute both to litter and to the soil solution. Cell walls and dead organisms also contribute to litter. Consumers are important in recycling primary saprotrophs back into litter and soil solution compartments. Respired CO2 from decomposition (not shown) also enters primary production through photosynthesis.

WASTE(g) (respired CO2)

WASTE(l) (excreted)

WASTE(s) (cell wall, cuticle, excreted)

Fig. 6.3. Litter transformation by saprotrophs. Individual saprotrophs ingest food (prey and nutrients) in order to grow (new biomass) and excrete the undigested portion (Waste(s)), the nitrogenous wastes (W^) and respired gases (Waste,)), and

WASTE(g) (respired CO2)

WASTE(l) (excreted)

WASTE(s) (cell wall, cuticle, excreted)

Fig. 6.3. Litter transformation by saprotrophs. Individual saprotrophs ingest food (prey and nutrients) in order to grow (new biomass) and excrete the undigested portion (Waste(s)), the nitrogenous wastes (W^) and respired gases (Waste,)), and release heat generated from metabolism.

recycling for photosynthesis and primary production. Until the recent impact of humans on ecosystems over the last 250 years, the release of soil CO2 and its uptake by photosynthesis were closely balanced (Schlesinger and Jeffrey, 2000). The balance is now disrupted, with the accumulation of an additional 7 X 1015 g C annually in the equation, mostly from burning fossil fuels and destruction of habitats. There have been many discussions of the global carbon budget as a result, to predict the impact of climate change on ecosystems, and on the rate of warming caused by the rate of an increase in CO2 (a greenhouse gas) in the atmosphere. In particular, soils are seen as important because they are more easily managed than the ocean or marine soil (Amundson, 2001; Fang et al., 2001; Pacala et al., 2001). The global approach to ecosystem analysis relies on two key observations: (i) that ecosystems are distributed into similar life zones based on similar mean annual precipitation and mean annual temperature (Holdridge, 1947; Whittaker, 1975); and (ii) that plant communities vary along climatic and SOM gradients (Post et al., 1982). These observations allow calculations of carbon pool fluxes within life zones and comparisons between ecosystems (Table 6.1, see Amundson, 2001). The cycling of organic matter does not occur independently for each element, even though they are often represented graphically independently (Figs 6.1, 6.4 and 6.5; Schlesinger, 1997).

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