Microbial Effects on the Fractionation of Cu and Zn in the Rhizosphere of Forest Soils.
Benoît Cloutier-Hurteau, Dépt de Géographie, Univ de Montreal, 520 Côte Sainte-Catherine, C.P. 6128 Succ. Centre-ville, Montréal, QC H3C 3J7, Canada, Sébastien Sauvé, Dépt de Chimie, Univ de Montréal, Case postale 6128 Centre-ville, Montréal, QC H3C 3J7, Canada, and François Courchesne, Dépt de Géographie, Univ de Montréal, 520 Côte Sainte-Catherine, C.P. 6128 Succ. Centre-ville, Montréal, QC H3C 3J7, Canada.
The transfer of metals from soils to plants is a fundamental flux in the biogeochemical cycle of forested ecosystem. This transfer is regulated by active processes in the rhizosphere, a large fraction of which are controlled by microorganisms. Unfortunately, our knowledge of the influence of microorganisms on metal distribution and speciation in soils is sparse. This gap in knowledge on rhizosphere processes precludes a sound understanding of the fate of metals in terrestrial ecosystems. Studies have identified several microbial mechanisms regulating the mobility of metals such as: biosorption, solubilization and redox reactions (Gadd, 2004, Geoderma: 109-119). Recent in situ studies in forested areas have shown that microbial activity is a key factor in controlling the distribution of Cu species in bulk and rhizospheric soil samples (Cloutier-Hurteau, 2005, 8th ICOBTE, Adelaide, Australia). Nevertheless, few attempts were made to establish in situ the functional links between microbial activity and the distribution of metals species in forest soils. In this context, the objectives of this study are 1) to contrast the rhizosphere and the bulk components of forest soils with respect to chemical and microbiological properties and 2) to evaluate how microbial activity in the rhizosphere affects the speciation of Cu and Zn in the solid phase and in the water extract. Soils were collected in the Rouyn-Noranda area (~ 600 km north west of Montreal, Canada) in August 2004 and in the Monteregie area (~ 80 km south of Montreal) in August 2005. At Rouyn-Noranda, three sites located at 0.5, 2 and 8 km from the Horne smelter were selected whereas in the Monteregie, three sites situated near an airport, a cement plant and a steel-work plant were selected, to insure the presence of sufficient of metals loads in soils. At every site, the rhizosphere and bulk components were sampled in clay soils (Luvisols) under Populus tremuloides. The separation of rhizospheric materials from the soil matrix was performed in the field by hand agitation of roots (Seguin, 2004, Plant and Soil: 1-17). The characterization of microbial properties was established by measuring total microbial biomass C and N, active microbial biomass C, the microbial C/N ratio, the dehydrogenase activity and the urease activity. We also measured the fractionation of Zn using two types of extractants: water and HNO3-HCl. On the water extract, we analyzed the water soluble Zn by ICP-AES and the labile Zn by DPASV. Results from discriminant analysis of the Rouyn-Noranda soil samples reveal a clear statistical contrast between rhizosphere and bulk components with respect to chemical and microbial properties (p < 0,0001). This contrast is mostly controlled by biological variables, water-soluble Zn and free-ion Cu. Also, previous results from Rouyn-Noranda on the links between microorganisms and Cu species showed a strong positive correlation between urease activity, microbial C/N ratio, organic carbon and Cu2+, on the one hand, and between dissolved organic carbon and water soluble Cu, on the other hand (Cloutier-Hurteau, 2005, 8th ICOBTE, Adelaide, Australia). These results demonstrate that the microbial activity promotes the mobility of Cu in the soil and that it explains 43% of the distribution of water-soluble Cu and 21% of the distribution of Cu2+ in rhizosphere and bulk samples. We anticipate the existence of a similar link between microorganisms and Zn, although Zn in soil is less strongly associated to organic matter than Cu. These results highlight the importance of considering the rhizosphere as a discrete soil environment and its role as a hot spot in the biogeochemichal cycle of forest ecosystem. Also, results reveal that microorganisms have a strong influence on metal speciation and stress the importance of the microbially-mediated processes in the rhizosphere and in the biogeochemical cycle of metals in forest soils.