Constraints to Synergistic Fe Mobilization from Calcareous Soil by a Phytosiderophore and a Reductant

Dissolved organic carbon diminishes manganese oxide-driven oxidation of chromium

Oxidation of chromium (Cr)-bearing minerals by manganese (Mn) oxides is viewed as the dominant mechanism controlling geogenic production of Cr(VI) and its contamination of groundwater. This process may be modulated by other chemical constituents found in the natural environment, but such confounding factors have not been quantified. Here, we evaluated the mechanism of Cr(III) oxidation by mixed-valence Mn oxide in the presence of citric and gallic acids, two natural organic matter (NOM) constituents commonly found in the soil environment. Incubation experiments showed that each organic acid enhanced solubilization of Cr(III) and Mn over controls without organic addition but increasing organic acid concentration decreased production of Cr(VI), with approximately 8.5 times less Cr(VI) produced in the citric acid than gallic acid experiments. X-ray absorption spectroscopy showed that negligible Cr(VI) was present in solid-phase reaction products, regardless of treatment. Geochemical modeling revealed that in the citric acid experiments, unprotonated Cr(III)-citrate was the dominant organo-metallic complex in solution, while (CrOH)2+ distribution positively correlated with concentrations of Cr(VI) produced. Collectively, these results illustrate how NOM can modify expected chemical pathways driving Cr cycling, and such mechanistic information should be better integrated into models predicting Cr redox dynamics and availability in the environment.

Stable Cu Isotope Ratios Show Changes in Cu Uptake and Transport Mechanisms in Vitis vinifera Due to High Cu Exposure

Even though copper (Cu) is an essential plant nutrient, it can become toxic under certain conditions. Toxic effects do not only depend on soil Cu content, but also on environmental and physiological factors, that are not well understood. In this study, the mechanisms of Cu bioavailability and the homeostasis of Vitis vinifera L. cv. Tannat were investigated under controlled conditions, using stable Cu isotope analysis. We measured Cu concentrations and δ⁶⁵Cu isotope ratios in soils, soil solutions, roots, and leaves of grapevine plants grown on six different vineyard soils, in a 16-week greenhouse experiment. The mobility of Cu in the soil solutions was controlled by the solubility of soil organic matter. No direct relationship between Cu contents in soils or soil solutions and Cu contents in roots could be established, indicating a partly homeostatic control of Cu uptake. Isotope fractionation between soil solutions and roots shifted from light to heavy with increasing Cu exposure, in line with a shift from active to passive uptake. Passive uptake appears to exceed active uptake for soil solution concentrations higher than 270 μg L^-1. Isotope fractionation between roots and leaves was increasingly negative with increasing root Cu contents, even though the leaf Cu contents did not differ significantly. Our results suggest that Cu isotope analysis is a sensitive tool to monitor differences in Cu uptake and translocation pathways even before differences in tissue contents can be observed.

Plasticity, exudation and microbiome-association of the root system of Pellitory-of-the-wall plants grown in environments impaired in iron availability

The investigation of the adaptive strategies of wild plant species to extreme environments is a challenging issue, which favors the identification of new traits for plant resilience. We investigated different traits which characterize the root-soil interaction of Parietaria judaica, a wild plant species commonly known as "Pellitory-of-the-wall". P. judaica adopts the acidification-reduction strategy (Strategy I) for iron (Fe) acquisition from soil, and it can complete its life cycle in highly calcareous environments without any symptoms of chlorosis. In a field-to-lab approach, the microbiome associated with P. judaica roots was analyzed in spontaneous plants harvested from an urban environment consisting in an extremely calcareous habitat. Also, the phenolics and carboxylates content and root plasticity and exudation were analyzed in P. judaica plants grown under three different controlled conditions mimicking the effect of calcareous environments on Fe availability: results show that P. judaica differentially modulates root plasticity under different Fe availability-impaired conditions, and that it induces, to a high extent, the exudation of caffeoylquinic acid derivatives under calcareous conditions, positively impacting Fe solubility.