Influence of microorganisms on Cu speciation in the rhizosphere of forest soils

Our study is one of the first attempts to document the copper (Cu) speciation in the rhizosphere of forest soils and to determine the importance and the influence of the microbial community on Cu speciation. In order to do this, bulk and rhizosphere samples were collected from field sites located close to industrial facilities. The rhizosphere materials were sampled under trembling aspen (Populus tremuloides Michx.) and separated from the bulk soils in the field. A characterization of the microbial populations was made by measuring microbial biomass C and N, urease and dehydrogenase activities. In soil water extracts, chemical properties were also measured, as well as total water-soluble Cu concentration (CuWS) and free-ion Cu activity (Cu2+). The residual Cu speciation was obtained by modelling, using MINEQL+ 4.5. In all cases, the Cu speciation was dominated by organic forms of Cu, the proportion of which increases with increasing pH. The reverse pH effect was observed for Cu2+. Moreover, almost systematically higher concentrations for all Cu variables were reported in the rhizosphere as compared to the bulk soils. The results also showed that microbial variables explained 22% of the distribution of CuWS and Cu2+ in bulk samples, a proportion that reached 61.5% in rhizospheric samples. In the rhizosphere, relationships between pH, microbial biomass N and Cu2+ indicated that microorganisms influenced Cu by modifying the pH of the solution through nitrogen assimilation. Furthermore, links found between urease activity, biomass variables, solid- and liquid-phase organic carbon and CuWS suggested that microbial mineralization could partly supply Cu to the solution fraction of the rhizosphere through root decay. This study reveals that microorganisms have a strong influence on Cu speciation in the rhizosphere of forest soils and suggests that a realistic understanding and representation of Cu dynamic in the rhizosphere must take microbial activity into account. Further investigations are needed to identify and establish precisely how microbial processes impact on Cu speciation.