Carbon , rhizodeposition

The input of carbon into soil via rhizodeposition and the decay of roots has been quantified in several studies using either pulse or continuous CO -labeling techniques (4,7,13-15), and estimates of carbon rhizodeposition vary considerably. 

III. CARBON DYNAMICS IN THE RHIZOSPHERE A. Input Rates of Rhizodeposition... 

Input rates of organic C into the soil system are hard to quantify, particularly for natural ecosystems and to a lesser extent for agricultural ecosystems. Whereas quantity and quality of carbon inputs via litter fall and plant residues after harvest might be directly measurable, inputs via roots and rhizodeposition are more difficult to assess. 

The quantification of gross root production, rhizodeposition, microbial assimilation, and the production of organic materials in soil has made increasing progress ever since stable ( C) and radioactive ( C) carbon isotopes have been used (see Chap. 12). Measurements of soil organic matter dynamics without these isotopes are difficult due to the large amount present as compared to the smaller rates of input. 

Several authors have applied in situ pulse labeling of plants (grasses and crops) with C-CO2 under field conditions with the objective of quantifying the gross annual fluxes of carbon (net assimilation, shoot and root turnover, and decomposition) in production grasslands and so assess the net input of carbon (total input minus root respiration minus microbial respiration on the basis of rhizodeposition and soil organic matter) and carbon fixation in soil under ambient climatic conditions in the field. 

Recurrent is the lack of adequate techniques to assess carbon flows through the plants and microbes into soil organic matter (151). Most important is the development of techniques and protocols to separate rhizosphere from nonrhizosphere soil as well as possibly to facilitate analyses of soil carbon dynamics. The use of carbon isotopes, and, where possible, application of double labeling with C and C, seems inevitable in order to separate the contribution of different substrates to the formation of the soil organic matter pool and to get to an understanding of the ecological advantage of exudates and rhizodeposits. 

Loss of carbon compounds from roo(s, or rhizodeposition, is the driving force for the development of enhanced microbial populations in the rhizosphere in comparison with the bulk soil. Although rhizodeposition is a general phenomenon of plant roots, the compounds lost from different species or even cultivars can vary markedly in quality and quantity over time and space. 

Such differences in the amount and type of rhizodeposition that occur on the root with time result in concomitant variations in microbial populations in the rhizosphere, both within the root (endorhizosphere), on the surface of the root (rhizoplane), and in the soil adjacent to the root (ectorhizosphere). The general microbial population changes and specific interaction of individual compounds from specific plants or groups of plants with individual microbial species are covered in detail elsewhere (Chap. 4). Consequently, this chapter is restricted to consideration of methodologies used to study carbon flow and microbial population dynamics in the rhizosphere, drawing on specific plant-microbe examples only when required. 

Rather than measuring rhizodeposition in nutrient. solutions chemically, another approach has been to expose shoots to COi for a short period of time and to follow the spread of the through the plant, into the roots, and then into the nutrient solution. Kinetics of carbon flow and quantification of rhizodeposition can then be obtained (e.g.. Ref. 24). Advantages and limitations of this approach are discussed more fully in Sec. II.C. 

Other sand-based systems using COi pulse-chase procedures have been used to produce carbon budgets for Festuca ovina and Plantago lanceolata seedlings (30) and white lupin (Lupimis albiis) (31). Significantly, CO2 pulse labeling of proteoid roots of white lupin under phosphate-deficient conditions showed that high levels of dark fixation of COi by the roots took place and that 66% of this root-fixed carbon was exuded from the roots (31). Clearly, dark fixation of CO2 by roots and subsequent rhizodeposition is an area that deserves further study in the future. 

One practical aspect of the procedure for monitoring carbon flow following C labeling is the need to separate roots from the soil for analysis. Incomplete removal of roots can lead to an overestimation of rhizodeposition, but overzealous washing of soil may lead to leaching of " C or loss of fine roots. This problem has been examined in detail for wheat and barley, and procedures to correct for these errors have been developed (69). 

The loss of carbon compounds from roots can influence microbial populations in various ways. Since the presence of readily available carbon sources is thought to be the most limiting factor to microbial growth in soil (96), rhizodeposition acts at a gross level to stimulate microbial populations. This generates the rhizo-sphere effect (97), where the number of microorganisms in the rhizosphere (R)... 

Carbohydrate partitioning from shoot to roots can comprise a substantial proportion of plant-photosynthetic C02 fixation (20-70%), and 4-70% of this fraction can be released into the soil as organic rhizodeposition (Lynch and Whipps, 1990 Grayston et al., 1996). This is not only a significant loss of reduced carbon but can also contribute by 30-40% to the total input of soil-organic matter with considerable impact as a source of carbon and nitrogen for soil microorganisms. 

According to the mechanisms of release, organic rhizodepositions may be grouped into these major fractions lysates, leachates from sloughed-off cells, and dead tissues as a consequence of root turnover. In contrast, root exudates (2-10% of translocated carbon) are released from intact root cells either passively as diffusates or actively as excretions or secretions with specific functions (Grayston et al., 1996 Neumann... 

Organic compounds released from sloughed-off root cells and tissues are a major carbon source for rhizosphere microorganisms but may indirectly have an impact as microbial metabolites on nutrient availability and on exclusion of toxic elements in the rhizosphere (Brimecombe et al., 2007). Continuous root turnover is a general feature of plant development, and insoluble root debris may comprise 50-90% of total rhizodeposition (Darrah, 1991). 

As said above, plant root chemistry may also influence deeply alpine soil microorganism s biomass. It turns out that the particular chemical composition of exudates is a strong selective force in favour of bacteria that can catabolize particular compounds. Plants support heterotrophic microorganisms by way of rhizodeposition of root exudates and litter from dead tissue that include phenolic acids, flavonoids, terpenoids, carbohydrates, hydroxamic acids, aminoacids, denatured protein from dying root cells, CO2, and ethylene (Wardle, 1992). In certain plants, as much as 20-30% of fixed carbon may be lost as rhizodeposition (Lynch and Whipps, 1990). Most of these compounds enter the soil nutrient cycle by way of the soil microbiota, giving rise to competition between the myriad species living there, from microarthropods and nematodes to mycorrhiza and bacteria, for these resources (e.g. Hoover and Crossley, 1995). There is evidence that root phenolic exudates are metabolized preferentially by some soil microbes, while the same compounds are toxic to others. Phenolic acids usually occur in small concentration in soil chiefly because of soil metabolism while adsorption in clay and other soil particles plays a minor role (Bliun et al., 1999). However, their phytotoxicity is compounded by synergism between particular mixtures (Blum, 1996). 

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