Rhizosphere biological processes

The rhizosphere, which is characterized by distinct physical, chemical, and biological conditions (Figures 1.14 and 1.15), is the focal point of the potential role of higher plants in accelerating natural remediation in contaminated soils. Rhizosphere conditions are created by the plant roots and their microbial associations. Rhizospheric biogeochemical processes are influenced substantially by soil physical, mineralogical, chemical, and biological features and also by edaphic and climatic conditions. Special tools and techniques are required to study the characteristics and processes of the rhizosphere because of its limited areal extension (Wenzel et al., 2001). 

Huang, P. M., and Germida. J. J. (2002). Chemical and biological process in the rhizosphere metal pollutants. In Interactions Between Soil Particles and Microorganisms Impact on Terrestrial Ecosystem, ed. Huang, P. M. Bollag, J.-M., and Senesi, N., lUPAC Series on Analytical and Physical Chemistry of Environmental Systems, Vol. 8, Wiley, Chichester, West Sussex, England, 381-438. 

As summarized by Jacobson (1994), biological Fe(III) reduction will be more important than chemical reduction when amorphous Fe(IIl) oxides are plentiful and continually regenerated, or H2S production is low relative to the Fe(III) concentration. This first condition is likely to be met in the rhizosphere where radial O2 loss drives Fe oxide formation. The second condition will be met in low-salinity wetlands, or in saline systems with mineral (i.e., iron-rich) sediments. However, even chemical reduction of Fe(III) is ultimately due to microbes since the H2S that reduces the Fe is the result of a biological process, SO4" reduction (Megonigal et al., 2004). 

Therefore, the role of physicochemical-biological interfacial interactions in controlling the transformation, transport, fate, and toxicity of metals and metalloids in soil and surrounding environments, especially the rhizosphere, which is the bottleneck of contamination of the terrestrial food chain, deserves increasing attention. In this chapter we present an overview of this emerging and extremely important area of science, to advance our knowledge of the interface between physicochemical and biological reactions and processes in the environment. 

Furthermore, abiotic and biotic reactions are not independent but rather, interactive processes in soil environments. Interactions of abiotic and biotic processes are thus very important in governing the dynamics and fate of metals and metalloids in soils, especially at the soil-root interface. Abiotic and biotic interactions in the rhizosphere in influencing the stabilization of contaminants and the efficacy of ameliorants need to be investigated. The impact of physical, chemical, and biological interfacial interactions on risk assessment and management of metal and metalloid contamination and restoration of ecosystem health merits close attention. 

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. 

As a by-product, the weathering processes occurring in the rhizosphere have produced yellowish-coloured amorphous Ee-oxyhydroxides that revealed the thickness of the soil affected by root activity, where chemical, mineralogical and biological properties were changed with respect to the bulk. 

---