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Heavy metals (HMs) are abundant in the karst soils of Southwest China, posing significant health risks to millions of people. Iron (Fe) (hyr)oxides serve as critical carriers of HMs in these soils; however, the processes governing Fe oxide formation and transformation associated with HM accumulation during carbonate weathering in karst region is less understood. In this study, we present Fe isotope compositions from a carbonate-derived profile to investigate the major factors controlling Fe migration. In the saprolite layer, strong correlations between δ 56 Fe bulk and the proportions of extracted Fe NH2OH.HCl or Fe residue fractions suggest that the formation of goethite and phyllosilicate may be responsible for variations in δ 56 Fe bulk . The positive correlations between δ 56 Fe NH2OH.HCl values and HM concentrations in this layer suggest an enhanced capacity for HM fixation by goethite in these soils. In contrast, the fractionation of Fe isotopes in the soil layer appears to be influenced by vegetation, as indicated by the correlation between total organic carbon and δ 56 Fe bulk . The negative correlations between δ 56 Fe bulk values and HM concentrations in the soil layer likely indicate that vegetation litter enhances the retention capacity of Fe oxides for HMs. This study highlights the trajectory of Fe and its connection to HM accumulation in karst soil with high geological background levels., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier B.V. All rights reserved.)

Land-use change influences the fate of nutrient elements, including iron (Fe), and then threaten soil security. In this study, Fe forms and stable isotope composition (δ 56 Fe) in soils were investigated to identify the variations in the processes of Fe cycling during agricultural abandonment in a karst region of Southwest China. Soil δ 56 Fe compositions varied from -0.05‰-0.02‰ in croplands, 0.05‰-0.12‰ in abandoned croplands, to 0.30‰-0.80‰ in the native vegetation lands. In the croplands, Fe oxidation-precipitation process is considered as the main contributor to Fe migration and isotope fractionation, leading to a relatively enrichment of heavier Fe isotope in deeper soil layer. In the abandoned croplands and native vegetation lands, Fe isotope in the organic-rich layer (0-10 cm) was significantly lighter than that in subsurface layer (20-30 cm), mainly due to the recovery of soil organic carbon (SOC) and macro-aggregate after cropland abandonment. Moreover, the eluviation process mainly caused a decrease in soil Fe contents and enrichment of heavy Fe isotope in deeper soils (below 40 cm). The positive correlation between oxidized Fe and SOC contents suggested the accumulation of mobile Fe in soils after agricultural abandonment, which is beneficial for Fe uptake and assimilation by plants. This study suggests that agricultural abandonment significantly reduce soil Fe leaching loss and improve plant Fe supply by SOC accumulation in surface soil, which gives an environmental implication for the management of soil nutrients., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier Inc. All rights reserved.)

Iron (Fe) is an essential nutrient for living organisms and Fe deficiency is a worldwide problem for the health of both rice and humans. Zinc (Zn) contamination in agricultural soils is frequently observed. Here, we studied Fe isotope compositions and transcript levels of Fe transporter genes in rice growing in nutrient solutions having a range of Zn concentrations. Our results show Zn stress reduces Fe uptake by rice and drives its δ 56 Fe value to that of the nutrient solution. These observations can be explained by the weakened Fe(II) uptake through Strategy I but enhanced Fe(III) uptake through Strategy II due to the competition between Zn and Fe(II) combining with OsIRT1 (Fe(II) transporter) in root, which is supported by the downregulated expression of OsIRT1 and upregulated expression of OsYSL15 (Fe(III) transporter). Using a mass balance box model, we also show excess Zn reduces Fe(II) translocation in phloem and its remobilization from senescent leaf, indicating a competition of binding sites on nicotianamine between Zn and Fe(II). This study provides direct evidence that how Zn regulates Fe uptake and translocation in rice and is of practical significance to design strategies to treat Fe deficiency in rice grown in Zn-contaminated soils., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

Microbial activity is often invoked as a direct or indirect contributor to the precipitation of ancient chemical sedimentary rocks such as Precambrian iron formations (IFs). Determining a specific metabolic pathway from the geological record remains a challenge, however, due to a lack of constraints on the initial conditions and microbially induced redox reactions involved in the formation of iron oxides. Thus, there is ongoing debate concerning the role of photoferrotrophy, that is the process by which inorganic carbon is fixed into organic matter using light as an energy source and Fe(II) as an electron donor, in the deposition of IFs. Here, we examine ~2.74-Ga-old Neoarchean IFs and associated carbonates from the Carajás Mineral Province, Brazil, to reconstruct redox conditions and to infer the oxidizing mechanism that allowed one of the world's largest iron deposits to form. The absence of cerium (Ce) anomalies reveals that conditions were pervasively anoxic during IF deposition, while unprecedented europium (Eu) anomalies imply that Fe was supplied by intense hydrothermal activity. A positive and homogeneous Fe isotopic signal in space and time in these IFs indicates a low degree of partial oxidation of Fe(II), which, combined with the presence of 13 C-depleted organic matter, points to a photoautotrophic metabolic driver. Collectively, our results argue in favor of reducing conditions during IF deposition and suggest anoxygenic photosynthesis as the most plausible mechanism responsible for Fe oxidation in the Carajás Basin., (© 2021 John Wiley & Sons Ltd.)

In paddy soils, iron (Fe) forms are highly influenced by the seasonal redox changes and leave detectable isotope signals because of fractionation between different Fe forms. Here, we present Fe concentrations and Fe isotope compositions (expressed as δ 56 Fe values) in a paddy soil profile from Suzhou, China. Light Fe isotopes were enriched in two iron-accumulation layers (Br3 and G1) with high Fe concentrations. In particular, large shifts in both Fe concentrations and δ 56 Fe values were found at the Br2 and Br3 boundaries, showing fast and efficient transformation between these horizons. With sequential extraction, we show that Fe isotopes in the short-range-ordered Fe minerals and crystalline Fe oxides were lighter than those in the residual silicate minerals. Iron enriched in light isotopes was leached from the Ap horizon and subsequently moved to Br horizon, quickly precipitating there as Fe oxides.

The Earth's most severe ice ages interrupted a crucial interval in eukaryotic evolution with widespread ice coverage during the Cryogenian Period (720 to 635 Ma). Aerobic eukaryotes must have survived the "Snowball Earth" glaciations, requiring the persistence of oxygenated marine habitats, yet evidence for these environments is lacking. We examine iron formations within globally distributed Cryogenian glacial successions to reconstruct the redox state of the synglacial oceans. Iron isotope ratios and cerium anomalies from a range of glaciomarine environments reveal pervasive anoxia in the ice-covered oceans but increasing oxidation with proximity to the ice shelf grounding line. We propose that the outwash of subglacial meltwater supplied oxygen to the synglacial oceans, creating glaciomarine oxygen oases. The confluence of oxygen-rich meltwater and iron-rich seawater may have provided sufficient energy to sustain chemosynthetic communities. These processes could have supplied the requisite oxygen and organic carbon source for the survival of early animals and other eukaryotic heterotrophs through these extreme glaciations., Competing Interests: The authors declare no competing interest.