Relevance of Rhizosphere Research to the Ecological Risk Assessment of Trace Metals in Soils

ABSTRACT

The chemical, mineralogical, and microbial properties of the rhizosphere of a range of forested ecosystems were studied to identify the key processes controlling the distribution and fate of trace metals at the soil–root interface. The results of our research indicate that: (1) the rhizosphere is a soil microenvironment where properties (e.g., pH, organic matter, microbes) and processes (nutrient and water absorption, exudation) differ markedly from those of the adjacent bulk soil; (2) the rhizosphere is a corrosive medium where the weathering and neoformation of soil solid phases are enhanced; (3) the concentrations of solid-phase and water-soluble trace metals like Cd, Cu, Ni, Pb, and Zn are generally higher in the rhizosphere as shown by both macroscopic and microscopic approaches; (4) a larger fraction of water-soluble metals is complexed by dissolved organic substances in the rhizosphere; and (5) soil microorganisms play, either directly or indirectly, a distinct role on metal speciation, in particular Cu and Zn, in the rhizosphere. These results improve our capacity to estimate metal speciation and bioavailability at the soil–root interface. Furthermore, the research emphasizes the crucial physical position occupied by the rhizosphere with respect to the process of elemental uptake by plants and its key functional role in the transfer of trace metals along the food chain. We conclude that the properties and processes of the rhizosphere should be viewed as key components of assessments of the ecological risks associated with the presence of trace metals in soils.

Key Words:

• rhizosphere
• trace metals
• forest soils
• speciation
• bioavailability
• roots

ACKNOWLEDGMENTS

The authors gratefully acknowledge the financial support of the National Sciences and Engineering Research Council of Canada (NSERC) and of the NSERC Research Network Metals in the Human Environment (MITHE-RN). A full list of MITHE-RN sponsors is available at www.mithe-rn.org. We also thank Alain Dufresne, George R. Gobran, Nathalie Kruyts, Hélène Lalande, Pascale Legrand, Sheila Macfie, Ron Martin, Steven Naftel, and Véronique Séguin for their contributions to this research program. The manuscript was improved by the comments of three anonymous referees.

Notes

^†pH _H2O = pH measured in water (1:10 soil to solution ratio); org C = organic carbon in the solid phase; CEC = cation exchange capacity in cmol₍₊₎/kg; minerals in the clay fraction (< 2 μ m) are in order of relative abundance where Qz = quartz, Ve = vermiculite, Pl = plagioclase, Ch = chlorite, Am = amphibole, Mi = micas, Kf = K-feldspar, and Ex = expandable phyllosilicate.

^†Intensity of a the XRD-peak for a given mineral divided by the intensity of the hkl = 100 peak for quartz (d = 0.426 nm).

^‡Data from Skogaby are the mean and standard deviation (in parentheses) values for six sites.