Rhizosphere – A Unique Interface for Understanding the Fate of Trace Elements – the Example of Copper.
Philippe Hinsinger1, Valérie Chaignon1, Benoît Cloutier-Hurteau2, Jean-Yves Cornu1, P Legrand2, Aurélia Michaud1, Véronique Séguin2, and François Courchesne2. (1) UMR 1222 Rhizosphere & Symbiose INRA-ENSAM, 2 Place Pierre Viala, Montpellier, 34060, France, (2) Département de Géographie. Université de Montreal, 520 Côte Sainte-Catherine, C.P. 6128 Succ. Centre-ville, Montréal, QC H3C 3J7, Canada
The rhizosphere is defined as the volume of soil around roots that is influenced by root activity. As such, it can be considered as a unique site for strong interfacial interactions between soil, living roots and microorganisms. The aim of the present paper is to give an overview of the range of interactions that are involved in changes in copper (Cu) speciation in the rhizosphere and which ultimately govern the bioavailability of Cu to plants. Changes of Cu concentration in the soil solution sampled in the rhizosphere of various plants have been reported in several works. For example, a significant increase in water-extractable Cu was reported in the rhizosphere of trees sampled in situ in forest soils that had been contaminated by atmospheric deposition of Cu from nearby smelting activity in Canada,. This could be the result of the decrease of rhizosphere pH and/or increase in dissolved organic carbon that was concomitantly observed, as a result of the exudation activity of tree roots. Conversely, in another study conducted in acidic, vineyard soils that had been contaminated by Cu due to the heavy and repeated use of Cu-based fungicides, it was shown that CaCl2-extractable Cu and soil solution Cu (collected by Rhizon samplers) decreased in the rhizosphere of pot-grown crop plants. In this case, it was shown that this decrease in Cu solubility could be fully explained by the root-induced increase in rhizosphere pH that was simultaneously measured. Changes of Cu speciation in the rhizosphere has been little studied, although the above-mentioned biogeochemical interactions point to the likely occurrence of such additional changes as a consequence of root and associated microbial activities. This can be explained by the considerable technical difficulties associated to the measurement of the speciation of trace metals at the spatial resolution that is relevant to rhizosphere interactions. Nevertheless, recent works based on the determination of free Cu-ion activities by ion-selective electrodes in water extracts of rhizosphere versus bulk soil samples has proved an efficient tool to further our understanding of root-induced changes of Cu speciation. It was found in the previous example of forest trees growing near a smelting area that the proportion of free Cu-ion in water-extractable Cu was reduced in the rhizosphere compared to the bulk soil. This suggests that greater complexation of Cu occurred in the rhizosphere, most probably due to organic ligands exuded by tree roots or produced by rhizosphere microorganisms. Changes of Cu bioavailability as a consequence of changes of either rhizosphere pH or exudation of organic ligands has been reported in several of the above works. In the previous works that reported a root-induced increase in pH in the rhizosphere of crop plants grown in an acidic soil, it was found that this phenomenon resulted in a decreased bioavailability of Cu. The most remarkable change in Cu bioavailability occurring in the rhizosphere is that reported to occur as a consequence of Fe-deficiency in grass species and of the subsequent secretion of efficient organic ligands called phytosiderophores. It was indeed found that Fe-deficient wheat plants grown in a calcareous, vineyard soils that had been contaminated by Cu due to the heavy and repeated use of Cu-based fungicides, took up 3-4 fold more Cu and secreted 3-4 fold more phytosiderophores than Fe-sufficient plants. These phytosiderophores have proved very efficient at complexing a range of trace metals,especially Cu. However their direct involvement in changes in Cu speciation and bioavailability still needs to be demonstrated. In addition, such ligands are found only in grasses, while organic ligands exuded by other plant species would hardly compare in terms of metal-complexing ability. The potential role of microbial-borne ligands which are much more diverse and include a range of efficient chelating agents such as the large family of siderophores would deserve being studied in the rhizosphere in future works.