| By controlling metal uptake by plants, the rhizosphere strongly influences the distribution of metals in forested ecosystems. Studies on the biogeochemistry of metals in the rhizosphere provided detailed accounts of the distribution of metals in the solid phase of this soil microenvironment. However, very few studies have investigated the speciation of metals in the soil solution despite the fact that it represents the preferential pathway of metal uptake by plants. Similarly, although the effects of roots on the biogeochemistry of metals in the rhizosphere have been extensively studied, the influence exerted by soil microorganisms has been comparatively neglected.;The thesis is composed of two chapters reviewing the latest knowledge published on this topic, of one chapter describing the methodology used for this work and of four result chapters presented as research papers. The first article describes the modelling approach used to compute the speciation of metals. The following three papers present and discuss the results of this research. The speciation results revealed that Cu is largely present in organically-complexed forms, that the speciation of Zn is dominated by the free-ion form or by organic complexes while Al is mostly found as inorganic complexes. The speciation data further revealed that the rhizosphere was systematically enriched in bioavailable metal species as compared to bulk soil. This suggested that the processes acting in the rhizosphere promoted the formation of bioavailable metal species.;Moreover, links between microbial variables and metals were also studied and indicated that the rhizospheric microorganisms exerted a significant influence on the distribution of bioavailable metals species and that this influence was larger in rhizosphere than in the bulk soil. Two microbially-driven processes were proposed to explain their influence on the bioavailability of metals in the rhizosphere. The most important microbial process was the pH change induced by the assimilation of nitrogen. This process strongly impacted on the pH of the rhizosphere and, thus, on the speciation of metals. The second process was the microbially-induced decay of soil organic matter, in particular of dead roots, through which the metals sequestered in the soil organic matter of the rhizosphere were mobilized.;The original contributions of this thesis are two-fold. First, its focus on tree roots and field sampling gave us a unique opportunity to study the rhizosphere under relevant, meaningful and representative conditions. Secondly, the data produced during this study are unique. Indeed, very few studies have documented the speciation of dissolved metals in the rhizosphere as precisely as we did. Similarly, the influence exerted by microorganisms on metal speciation in this microenvironment was demonstrated here for one of the first time. Acquiring these key informations significantly improved our understanding of the processes controlling the speciation of metals in the rhizosphere, especially by emphasizing the crucial role played by microorganisms in this microenvironment. These informations are helpful to estimate precisely the metal uptake by plants and to assess the ecological and human risk associated to the transfer of metals from soil to the vegetation.;To address this gap in knowledge, the objectives of this thesis were defined as to (1) document the liquid-phase speciation of copper (Cu), zinc (Zn) and aluminum (Al) in the rhizosphere and bulk forest soils, (2) quantify the influence of microorganisms on the speciation of Cu, Zn and Al in these soils and (3) identify the microbial variables and processes controlling the speciation of these metals in the rhizosphere. To achieve these objectives, rhizosphere and bulk soils were sampled under 18 trembling aspen trees ( Populus tremuloides Michx) equally distributed at six field sites presenting contrasted soil properties. On each soil sample, biological and chemical properties as well as the labile and bioavailable Cu, Zn and Al were measured whereas geochemical modelling was used to speciate each metal studied.;Keywords. biogeochemical cycle, forest soils, trace metals, rhizosphere, pH, microbial biomass, differential pulse anodic stripping voltammetry, ion-selective electrode, MINEQL+ 4.5, multivariate statistical analyses. |
