Looking at the root-soil system as a whole, what is the totality of the resources involved, and how are these resources allocated under various conditions of stress and soil type?
Several reviewers (Coleman, 1976; Coleman et al., 1983; Fogel, 1985, 1991; Martin and Kemp, 1986; Cheng et al., 1993; Cheng, 1996; Kuzyakov, 2002) have noted that from 20 to 50% more carbon enters the rhizosphere from root exudates and exfoliates (sloughed cells and root hairs) than actually is present as fibrous roots at the end of a growing season. This was determined in a series of experiments using 14C as a radiotracer of the particulate and soluble carbon (Shamoot et al., 1968; Barber and Martin, 1976). In fact, the mere change from a hydroponic medium to a sand medium was enough to double the amount of labile carbon as an input to the medium. This difference was attributed to the abrasion of roots against sand particles. In addition, the root-rhizosphere microflora has the potential to act as a sizable carbon sink (Wang et al., 1989; Helal and Sauerbeck, 1991), which can double the losses to soil as well. This is convincing proof that the combined below-ground system—roots, microbes, soil, and fauna—is governed by source-sink relationships, just as are intact plants (i.e., roots and shoots).
Extensive amounts and complexities of carbon compounds are elaborated in the rhizosphere (Rovira et al., 1979; Kilbertus, 1980; Foster et al., 1983; Foster, 1988; Lee and Foster, 1991; Cheng et al., 1993). The extent to which this exuded carbon is integral to root and rhizosphere function is of great interest to ecologists. Nitrogen-fixing bacteria residing in the rhizosphere and the release of their nitrogen to the plant can be stimulated by root exudates (Rao et al., 1998, cited in Jones et al., 2003). There are numerous direct and indirect positive and negative effects of carbon flows in the rhizosphere that encompass a wide array of symbiotic associations and trophic and biochemical interactions (Jones et al., 2003) (Figs. 2.5 and 2.6). Although the potential for rhizodeposi-tion-driven N2 fixation in the soil is small in comparison to inorganic and symbiotic fixation inputs, it may be of importance in nitrogen-limited ecosystems (Jones et al., 2003). The boundary layer between roots and soil—the so-called "mucigel" (Jenny and Grossenbacher, 1963)—is jointly contributed by microbes and root surfaces. Studies of the root tip and capsule components have been most informative about the roles of signal molecules that are exuded at subnutritional rates in soil. One of the key components involved is the border cells. These cells are lost from the root tip at a rate regulated by the root and secrete compounds that alter the environment of, and gene expression in, soil microorganisms and fauna (Farrar et al., 2003) (Fig. 2.7). These root-tip capsule components include high molecular weight (MW) mucilage secreted by the root cap, as well as cell-wall breakdown products resulting from the separation of thousands of border cells from each other and the root cap (see Fig. 2.7).
Much research has been conducted on ways to separate total CO2 efflux into that from microbial respiration from soil organic matter
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