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FIGURE 12.12 The influence of secondary compounds or acid-insoluble substances (%) on the rate of decomposition.

major structural components described above. Root exudates are the more easily decomposed simple carbohydrates (sugars) and amino compounds. Roots contribute mucilage from their growing tips, sloughed root cells, and exudates to soil. A significant portion of root exudates are nutrients, especially N (He et al., 2006). The amount of C entering soil as root exudates has been fiercely debated mainly due to the complexity of studying roots in situ. Complicating this debate is the role of mycorrhizas in contributing C. The majority of C allocated below ground is respired within 2 weeks by the roots, mycorrhiza, and rhizosphere organisms. Root exudates are important substrates that contribute to rhizosphere processes, but their role in C cycling is not well understood. Natural grasslands labeled with 14CO2 retained 52% of the assimilated C above ground and 36% in belowground structures (Milchunas et al., 1985). In a field tree labeling experiment, only 20% of assimilated 14CO2 was found in Populus eugenii roots 2 weeks following labeling regardless of season (Horwath et al., 1994). The contribution of roots, although varying widely among different species, is often underestimated, leaving uncertainty in C budgets that aim to understand sources of C that maintain and sequester soil C.

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Organic Gardeners Composting

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