Trace Element Biogeochemistry in the Rhizosphere

A focal point of entry of TEs into the food chain is the so-called plant rhizosphere. Generally, the rhizosphere-located at the root-soil interface-isCONTENTSIntroduction ......................................................................................................... 147 Biogeochemistry and Bioavailability of Trace Elements as Affected by Rhizosphere Processes ........................................................................................ 149Root-Induced Changes in Soil Water Content Affecting Trace Element Bioavailability.................................................................................. 149 Root-Induced Changes in pH ....................................................................... 150 Root-Induced Changes in Redox Conditions ............................................. 150 Complexation and Chelation of Trace Elements in Rhizosphere ............ 151Recent Experimental Evidence for Rhizosphere Effects on Trace Element Solubility and Chemical Speciation .................................................. 152 Alleviation of Trace Element Toxicity in Rhizosphere ................................... 165 Trace Element Transport and Leaching in Rhizosphere and Rootzone ...... 173 Summary .............................................................................................................. 175 References ............................................................................................................. 176de"ned as the zone of soil around roots that is affected by root activities (Hinsinger, 1998; Hinsinger et al., 2005). It is now well established that growing, active roots are able to modify their soil environment in terms of physical, chemical, biochemical, and biological properties and processes. The intensity and the extension of these changes into the bulk soil largely depend on the speci"c interaction between the plant species and its physiological status; the physical, chemical, and biological properties of the soil; and the considered rhizosphere process or characteristic (Hinsinger et al., 2005; Hinsinger and Courchesne, 2008; Hinsinger et al., 2009). Root activities and related rhizosphere processes can signi"cantly affect the chemical solubility and phytoavailability of both nutrients and pollutants in the rhizosphere, thus constituting an important control of the environmental fate of chemical elements in terrestrial ecosystems. Rhizosphere processes are known to result in element accumulation or depletion, to induce transport processes such as diffusion and mass ˆow, and result in changes in chemical speciation and solubility. Interactions with rhizosphere microorganisms and their activities may induce further changes in the behavior of chemical elements in the rhizosphere (Hinsinger, 1998; Hinsinger and Courchesne, 2008; Wenzel, 2009).