Rhizosphere processes

Root products, as defined by Uren and Reisenauer (17), represent a wide range of compounds. Only secretions are deemed to have a direct and immediate functional role in the rhizosphere. Carbon dioxide, although labeled an excretion, may play a role in rhizosphere processes such as hyphal elongation of vesicular-arbuscular mycorrhiza (39). Also, root-derived CO2 may have an effect on nonphotosynthetic fixation of CO2 by roots subject to P deficiency and thus contribute to exudation of large amounts of citrate and malate, as observed in white lupins (40). The amounts utilized are very small and, in any case, are extremely difficult to distinguish from endogenous CO2 derived from soil and rhizosphere respiration. 

During the last two decades, the functional characterization of plant root exudates involved in rhizosphere processes has attracted increased attention. The role of... 

In addition to the interactions between plants and microorganisms, a third factor, the soil, also plays a role in determining root exudation and the activity and diversity of rhizosphere microbial populations. In this section, physical and structural aspects of the soil are discussed in relation to their effects on root exudation and microbial populations. Consideration is also given to the role of agricultural management practices on rhizosphere processes. In addition, the role of other biotic factors, such as microfaunal predation, is discussed in relation to nutrient cycling in the rhizosphere. 

Table 4 Soil Sampling Procedures, Limitations, and Advantages of Some Experimental Soil-Plant Systems Adopted for Studying Rhizosphere Processes... 

T. S. Gahoonia and N. E. Nieksen, A method to study rhizosphere processes in thin soil layers of different proximity to roots. Plant Soil 755 143 (1991). 

Cheng W. and Johnson D. W. (1998) Elevated CO2, rhizosphere processes, and soil organic matter decomposition. Plant Soil 2(2), 167-174. 

Analysis of the Pb EXAFS data from the Fe-plaque revealed a Pb-0 interatomic distance of 2.4 A, which is inconsistent with published data for Pb sorbed to Fe-oxides (-2.27 A). Furthermore, optimized fitting of the second shell EXAFS function was obtained for C or N at a distance of 3.4 A, which is longer than previous reports for Pb bound humic material (Hansel et al. 2001). Based on fits that included a sample where Pb was bound to microbial biofilm it was concluded that the Pb was bound to a microbial biofilm-like material apparently intimately associated with the Fe-plaque. This finding demonstrates the important role of microorganisms in rhizosphere processes and the preferential binding of Pb to extracellular polymeric substances even in the presence of a highly reactive high surface area Fe-oxide phase. 

Zoysa, A.K.N., Eoganathan, P. and Medley, M.J. (1997) A technique for studying rhizosphere processes in tree crops soil phosphorus depletion around camellia (Camellia japonica L.) roots. Plant and Soil 190, 253-265. 

Contribution of rhizospheric processes to mineral weathering in forest soils... 

We recognize that the addition of apatite grains to the soil, even in bags, could have an effect on the proximal biological activity (Wallander, 2000). Moreover, despite our SEM observations confirming losses of mineral masses recorded for apatite, it is still difficult to conclusively establish the contribution of rhizosphere processes on apatite weathering as long as this effect is not quantified. 

Gobran, G.R., Clegg, S., Courchesne, F, 1998. Rhizospheric processes influencing the of forest ecosystems. In Breemen, N. van (Ed.), Plant Induced Soil Changes Processes and Feedbacks. Biogeochemistry 42, 107-120. 

Gorissen, A., Cotrufo, M.E., 1999. Elevated carbon dioxide effects on nitrogen dynamics in grasses, with emphasis on rhizosphere processes. Soil Sci. Soc. Am. J. 63,1695-1702. 

Lombi, E., Wenzel, W.W., Gobran, G.R., Adriano, D.C., 2001. Dependency of phytoavailability of metals on indigenous and induced rhizosphere processes a review. In Gobran, G.R., Wenzel,... 

Zhang, R, Shen, J., Li, L., Liu, X., 2002. An approach to study rhizosphere processes under major cropping systems in China. In 17th World Congress of Soil Science, 14-21 August 2002, Bangkok, Thailand, pp. 938-1-938-10. 

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