Rhizodeposition composition

In summary, the Py-FI mass spectrum shows a great diversity in the molecular rhizodeposit composition which could not be explained by previous chromatographic analyses of root exudates (e.g., Gransee and Wittenmayer, 2000). These focused mainly at the identification and quantification of a priori expected compounds (Fan et al., 2001). Therefore, Py-FIMS may contribute to the detection of previously unknown rhizodeposits and high-molecular-weight products of rhizodeposit interaction with genuine SOM compounds. 

The ability to change and control the composition of the nutrient solution and the relatively small size of the microcosms used enables manipulation of environmental variables and time-course studies of rhizodeposition to be made relatively easily. The influence of nutrient availability, mechanical impedance, pH, water availability, temperature, anoxia, light intensity, CO2 concentration, and microorganisms have all been examined within a range of plant species (9). A few examples to illustrate the continued interest in examining the effect of such variables on rhizodeposition in nutrient culture are given in Table 1. 

Freeze-dried DOM samples collected with the siphon-elution system (Kuzyakov and Siniakina, 2001) for the first time showed diurnal dynamics in the molecular-chemical composition of maize rhizodeposits (Kuzyakov et al., 2003). In a forthcoming study with maize, Melnitchouck et al. (2005) showed that amino acids, especially aspartic acid, asparagine, glutamic acid, phenylalanine, leucine and isoleucine contributed to the more intensive rhizodeposition during daytime than during nighttime. Furthermore, the maximum of thermal volatilization of peptides at low pyrolysis temperature in Figure 14.8 indicates the rhizodeposition or microbial formation of free amino acids rather than amino acids bound in peptides or trapped in soil humic substances. 

Figure 14.8. Thermograms for the volatilization of peptide-derived compounds in freeze-dried rhizodeposits leached from a soil cropped with maize after a daytime and a nighttime growth period and thermogram for the volatilization of L-glutamic acid. Reprinted from Leinweber, P., Jandl, G., Baum, C., Eckhardt, K.-U., and Kandeler, E. (2008). Stability and composition of soil organic matter control respiration and soil enzyme activities. Soil Biology and Biochemistry 40,1496-1505, with permission from Elsevier.

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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