The speciation of water-soluble Al and Zn in the rhizosphere of forest soils

Abstract

This study focuses on the relationships of dissolved Al and Zn speciation with microbial and chemical soil properties in the bulk and rhizosphere of forest soils. The soil components were sampled under Populus tremuloides Michx. at six sites located close to industrial facilities. Total water-soluble (Al_WS, Zn_WS) and reactive (Al_R, Zn_R) Al and Zn concentrations measured in soil water extracts, speciation data modeled by WHAM 6, chemical properties (pH, DOC, major cations and anions) and microbial properties (microbial biomass and enzyme activities) were measured on all soils. Enrichment in Al_R and Zn_R was observed in the rhizosphere compared to bulk soils. In a given soil material, the speciation of Al and Zn varied according to solution pH and Al–organic as well as Zn–organic complexes or Zn²⁺ were generally the dominant species. The factors controlling the Al_WS, Zn_WS, Al_R and Zn_R concentrations differed between soil components, shifting from strictly chemical in the bulk (78%) to interactions among microbial and chemical variables in the rhizosphere (87%). Results further indicate that organic matter and pH were significantly linked to these response variables in the rhizosphere. Involvement of rhizospheric microorganisms occurred via pH changes induced by either the microbial assimilation of nitrogen or through the release of metals during the mineralization of roots. Our results therefore suggest that microbial activity is an important component of the biogeochemistry of Al and Zn in the rhizosphere. The study further provides key information to improve the assessment of ecological risk associated to Al and Zn in forest soils.