Zn biogeochemical cycle in highly Zn-contaminated soil-plant systems

(eng) High zinc (Zn) concentration in soil can lead to the contamination of the hydrological system or to excessive uptake by plants. Therefore, it is crucial to identify the processes of Zn migration in highly Zn contaminated soil-plant systems. The scientific strategy of the present study was divided in two main parts: Firstly, we have confirmed that the Zn mobility in the five highly contaminated soils (a calcareous soil, a shale-derived soil, and three slag heap soils with distinct plant covers) predominantly depends on pH conditions. We have also shown that sulphate and nitrate release is correlated with the Zn release in the soil solution suggesting that the microbial activity might affect Zn mobilization. In addition, two plant species (Brassica napus and Lolium multiflorum) were cultivated in controlled conditions; the two major parameters controlling the Zn uptake by plants were: (a) the pH in the close vicinity of roots and (b) the total Zn concentrations in soils. Secondly, we aimed to identify the processes inducing Zn fractionation during the migration of Zn in the plant-soil systems. We have shown that: (a) Zn isotopic compositions (δ66Zn ) of the contaminated soils reflect the relative contributions of aerosols vs. slags, characterized by contrasted isotopic signatures; (b) the δ66Zn of the plant roots and shoots reflect the Zn isotopic composition of soils, and seem to be dependent on the density of negative charges in roots which would control the magnitude of Zn isotopic fractionation during Zn transport towards shoots; (c) the magnitude of Zn fractionation between roots and shoots is additionally correlated to the transpiration volume/total dry biomass ratio; finally, (d) the variations in δ66Zn through the soil profiles might be the result of Zn incorporation in plants and the return of dead plant materials since we found a strong correlation between isotopic signatures of bulk soils and proportion of total Zn included in organic matter.