Impact of temperature on the dynamics of organic matter and on the soil-to-plant transfer of Cd, Zn and Pb in a contaminated agricultural soil.

Predicting the soil-to-plant transfer of metals in the context of global warming has become a major issue for food safety. It requires a better understanding of how the temperature alters the bioavailability of metals in cultivated soils. This study focuses on one agricultural soil contaminated by Cd, Zn and Pb. DGT measurements were performed at 10, 20 and 30 °C to assess how the bioavailability of metals was affected by a rise in soil temperature. A lettuce crop was cultivated in the same conditions to determine if the soil-to-plant transfer of metals increased with a rise in soil temperature. A gradual decline in Cd and Zn bioavailability was observed from 10 to 30 °C, which was attributed to more intense complexation of metals in the pore water at higher temperatures. Together with its aromaticity, the affinity of dissolved organic matter (DOM) for metals was indeed suspected to increase with soil temperature. One main output of the present work is a model which satisfactorily explains the thermal-induced changes in the characteristics of DOM reported in Cornu et al. (Geoderma 162:65-70, 2011) by assuming that the mineralization of initial aliphatic compounds followed a first-order reaction, increased with soil temperature according to the Arrhenius law, and due to a priming effect, led to the appearance of aromatic molecules. The soil-to-plant transfer of Cd and Zn was promoted at higher soil temperatures despite a parallel decrease in Cd and Zn bioavailability. This suggests that plant processes affect the soil-to-plant transfer of Cd and Zn the most when the soil temperature rises.

Comparison of soil solution sampling techniques to assess metal fluxes from contaminated soil to groundwater.

The unsaturated zone plays a major role in elemental fluxes in terrestrial ecosystems. A representative chemical analysis of soil pore water is required for the interpretation of soil chemical phenomena and particularly to assess Trace Elements (TEs) mobility. This requires an optimal sampling system to avoid modification of the extracted soil water chemistry and allow for an accurate estimation of solute fluxes. In this paper, the chemical composition of soil solutions sampled by Rhizon® samplers connected to a standard syringe was compared to two other types of suction probes (Rhizon® + vacuum tube and Rhizon® + diverted flow system). We investigated the effects of different vacuum application procedures on concentrations of spiked elements (Cr, As, Zn) mixed as powder into the first 20 cm of 100-cm columns and non-spiked elements (Ca, Na, Mg) concentrations in two types of columns (SiO2 sand and a mixture of kaolinite + SiO2 sand substrates). Rhizon® was installed at different depths. The metals concentrations showed that (i) in sand, peak concentrations cannot be correctly sampled, thus the flux cannot be estimated, and the errors can easily reach a factor 2; (ii) in sand + clay columns, peak concentrations were larger, indicating that they could be sampled but, due to sorption on clay, it was not possible to compare fluxes at different depths. The different samplers tested were not able to reflect the elemental flux to groundwater and, although the Rhizon® + syringe device was more accurate, the best solution remains to be the use of a lysimeter, whose bottom is kept continuously at a suction close to the one existing in the soil.

Cadmium speciation assessed by voltammetry, ion exchange and geochemical calculation in soil solutions collected after soil rewetting.

Analytical techniques and speciation models have been developed to characterize the speciation of Cd in soil solution. They provide an estimate of operationally defined species of Cd that need to be compared, especially for soil solutions highly concentrated in organic matter as are the solutions collected after soil rewetting. This work deals with the comparison between the speciation of Cd measured by anodic stripping voltammetry (ASV) and ion exchange and the speciation of Cd calculated using Visual MINTEQ. The aim of this study was to quantify and explain the differences in Cd speciation observed between the three approaches. Cd speciation was assessed in soil solutions collected 4, 8, 24, 48, 96 and 144h after the rewetting of an air-dried contaminated soil. To optimize the computed speciation of Cd, other physico-chemical parameters were followed (e.g. pH, ionic strength and the concentrations of major anions, major cations and dissolved organic carbon) and a brief characterisation of dissolved organic matter (DOM) was performed. The discrepancy between model predictions and analytical measurements highlighted the need for caution in the interpretation of geochemical speciated data for Cd. The major result of this study was that a characterization of DOM based on its specific UV-absorbance at 254 nm improved the accuracy of model predictions. Another finding is that labile Cd complexes, even organic, may have been included in the electrochemically labile fraction of Cd measured by ASV.