Indirect Negative Effects

FIGURE 2.6. Schematic representation of the positive and negative direct and indirect effects of root exudates on plant growth (from Jones et al., 2003).

molecular-weight mucilage secreted by the root cap, (III) cell-wall breakdown products resulting from separation of thousands of border cells from each other and the root cap, (IV) other extracellular products secreted by the cap, (V) other extracellular products secreted by the border cells (from Farrar et al., 2003).

molecular-weight mucilage secreted by the root cap, (III) cell-wall breakdown products resulting from separation of thousands of border cells from each other and the root cap, (IV) other extracellular products secreted by the cap, (V) other extracellular products secreted by the border cells (from Farrar et al., 2003).

(SOM) and that from roots and rhizosphere microbial populations. Methods employed have been summarized into three broad categories (Hanson et al., 2000): component integration, root exclusion, and iso-topic approaches. Component integration entails separation of the constituent soil components involved in respiring CO2 (i.e., roots, sieved soil, and litter) followed by measurement of the specific rates of CO2 outputs from each component part (Coleman, 1973; Trumbore et al., 1995). The root exclusion method estimates root respiration indirectly by subtracting the soil respiration without roots from the soil respiration with roots (Anderson, 1973; Edwards, 1991). Isotope methods refer to the use of either radioisotopic 14C (Cheng et al., 1993; Horwath et al., 1994) or stable 13C isotopes to trace the origin of soil respiration (Andrew et al.,

1999; Robinson and Scrimgeour, 1995). Isotope methods have a significant advantage over the component integration and root exclusion methods because they permit researchers to partition carbon between rhizosphere respiration and SOM decomposition in situ, thus avoiding the effects of soil disturbance.

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