Trace elements and microbial activity in the rhizosphere

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Understanding the processes that regulate the solubility of trace elements (TE - As, Cd, Ce, Co, Cr, Cu, Ni, Pb, V, Y and Zn) in soils, their uptake by plants and transport towards waters bodies is a key societal challenge because this knowledge is vital to maintain the integrity of terrestrial and aquatic ecosystems, and to prevent health hazards to humans. We submit that the rhizosphere, because of the strategic position it occupies in soil:plant systems, is an ideal loci to study the processes controlling the speciation and bioavailability of TE in soils. The long-term objective of the research program is thus to establish the contribution of rhizospheric processes to the biogeochemistry of TE in terrestrial ecosystems. Based on our recent work, three pressing research questions were formulated: Question 1: To what extent is knowledge (observation, process, theory) on TE in the rhizosphere gathered from controlled environments valid in the field? Question 2: Could findings based on the study of a few tree species be generalized to a large spectrum of tree:soil systems? Question 3. What are the consequences of soil nutrient status on the speciation and bioavailability of TE in the rhizosphere of trees? The approach combines field studies of forested ecosystems and growth chamber experiments. Rhizosphere materials originating from a range soil parent materials and of vegetation canopies representative of the terrestrial environments found in north-eastern North America will be studied. Advances in the area of rhizosphere research are needed by our society and policy makers to anticipate the response of soils:plant systems to environmental changes and to limit the flow of TE along the food-chain in a context of sustainable food production.