Redistribution and isotope fractionation of endogenous Cd in soil profiles with geogenic Cd enrichment

Effects of climate and geochemical properties on the chemical forms of soil Cd, Pb and Cr along a more than 4000 km transect

Soil heavy metal speciation has received much attention for their different ecological and environmental effects. However, the effects of climate and soil geochemical properties on them in uncontaminated soils at macroscale were still unclear. Therefore, a transect more than 4000 km was chosen to study the effects of these factors on soil Cd, Pb and Cr forms. The results revealed that mean annual temperature and precipitation showed significant positive relations with the exchangeable and Fe-Mn oxide bound states of Cd, Pb and Cr, and residual Cr. And humidity and drought indexes were significantly positively correlated with their organic and carbonate bound forms, respectively. As for soil geochemical properties, pH displayed significant negative relationships with exchangeable, Fe-Mn oxide and organic bound Pb and Cr, and exchangeable Cd. Fe2O3 was significantly positively with the exchangeable and Fe-Mn oxide bound Cd, Pb and Cr, and residual Cr. And soil organic matter showed positive relations with organic bound Pb and Cr, and residual Cd and Cr, displayed negative relationships with carbonated bound Pb and Cr. Overall, climate and soil geochemical properties together affect the transformation and transport of heavy metals between different forms in uncontaminated soils.

A large and overlooked Cd source in karst areas: The migration and origin of Cd during soil formation and erosion

There is increasing concern regarding the substantial enrichment of Cd during the weathering of carbonate rocks and subsequent risks posed to the ecological environment and food security in karst areas. However, the incomplete understanding of Cd migration mechanisms and material sources restricts soil pollution control and land management. This study investigated the migration regulation of Cd during soil formation and erosion in karst areas. The results demonstrate that soil Cd concentration and bioavailability are both significantly higher in alluvium compared with those in eluvium. This increase is primarily attributed to the chemical migration of active Cd, rather than the mechanical migration of inactive Cd. Additionally, we analyzed the Cd isotopic characteristics of rock and soil samples. The isotopic composition of the alluvial soil was -0.18 ‰ ± 0.01 ‰, which is obviously heavier than the δ114/110Cd value of the eluvium (-0.78 ‰ ± 0.06 ‰). The Cd isotopic fingerprint revealed that the active Cd in the alluvium of the study profile was probably derived from the corrosion of carbonate rocks rather than by eluviation of the eluvium. Moreover, Cd tends to occur in soluble mineral components of carbonate rocks rather than in residues, which suggests that carbonate weathering has a great potential to release active Cd into the environment. It is estimated that the Cd release flux caused by carbonate weathering is 5.28 g Cd km-2 yr-1, accounting for 9.30 % of the anthropogenic Cd flux. Therefore, the corrosion of carbonate rocks is a substantial natural Cd source and poses significant potential risks to the ecological environment. It is suggested that the contribution of Cd from natural sources should be considered during ecological risk assessments and studies of the global Cd geochemical cycle.

Fate of Cd during mineral transformation by sulfate-reducing bacteria in clay-size fractions from soils with high geochemical background

Sulfate-reducing bacteria (SRB) can immobilize heavy metals in soils through biomineralization, and the parent rock and minerals in the soil are critical to the immobilization efficiency of SRB. To date, there is little knowledge about the fate of Cd associated with the parent rocks and minerals of soil during Cd immobilized by SRB. In this study, we created a model system using clay-size fraction of soil and SRB to explore the role of SRB in immobilizing Cd in soils from stratigraphic successions with high geochemical background. In the system, clay-size fractions (particle size < 2 µm) with concentration of Cd (0.24-2.84 mg/kg) were extracted from soils for bacteria inoculation. After SRB reaction for 10 days, the Cd fraction tended to transform into iron-manganese bound. Further, two clay-size fractions, i.e., the non-crystalline iron oxide (Fe-OX) and the crystalline iron oxide (Fe-CBD), were separated by extraction. The reaction of SRB with them verified the transformation of primary iron-bearing minerals into secondary iron-bearing minerals, which contributed to Cd redistribution. This study shows that SRB could exploit the composition and structure of minerals to induce mineral recrystallization, thereby aggravating Cd redistribution and immobilization in clay-size fractions from stratigraphic successions with high geochemical background.

Protective effects of polymer amendment on specific metabolites in soil and cotton leaves under cadmium contamination

Polymer materials have great potential for soil heavy metal contamination remediation, but the metabolic mechanism by which polymer amendments regulate the responses of soil-plant systems to cadmium (Cd) stress is still unclear. To clarify the metabolic mechanism by which a self-developed soluble polymer amendment (PA) remediates Cd contamination in cotton fields, the common and differential metabolites in soil and cotton leaves were analyzed during the critical period of cotton growth (flowering and bolling stage) in a field experiment. The results showed that Cd stress increased Cd concentration in the soil-cotton system, and reduced enzyme activity in soil and cotton leaves. Besides, Cd stress also reduced the abundance of α-linolenic acid in soil and the abundance of 2-Oxoarginine and S-Adenosylmethionine in cotton leaves. These ultimately led to reductions in weight, boll number, yield, and fiber elongation. However, the application of PA to the Cd-contaminated soil significantly reduced the soil exchangeable Cd (Ex-Cd) concentration by 41.43%, and increased the boll number, yield, and fiber strength by 14.17%, 21.04%, and 19.89%, respectively compared with the Cd treatment. The results of metabolomic analysis showed that PA application mainly affected the Nicotinate and nicotinamide metabolism pathway, Lysine degradation pathway, and Arginine and proline metabolism pathway in cotton leaves and soil. Besides, in these metabolic pathways, succinic acid semialdehyde of cotton leaves, saccharopine of soil, and S-Adenosylmethionine of soil and cotton had the most significant response to PA application. Therefore, the application of PA to Cd-contaminated soil can increase soil and cotton leaf enzyme activity and cotton yield (boll number and seed cotton yield) and quality (fiber strength), and maintain soil-plant material balance by regulating the distribution of Cd ions and key metabolites in the soil-cotton system. This study will deepen our understanding of the metabolic mechanism of PA remediating Cd-contaminated cotton fields, and provide a technical reference for the remediation of heavy metal contamination in drip-irrigated cotton fields in arid areas.

Zn isotope signatures in soil FeMn nodules with karst high geochemical background

Zn isotope has the potential to be used as an environmental tracer, due to its role in fingerprinting specific sources and processes. However, few studies have focused on Zn isotope system in terrestrial ferromanganese (FeMn) nodules, which is important on understanding the behaviors of Zn in soils. In this study, we analyse the isotopic composition in soil FeMn nodules and surrounding materials from a typical karst region in Guangxi Province, southwestern China and use advanced synchrotron-based methods to characterize Zn speciation. The Zn isotope compositions of the FeMn nodules range from 0.09 to 0.66 ‰, with an average value of 0.24 ‰. Pb isotope fingerprinting reveals that the major material sources contributing to the FeMn nodules are the surrounding soil (δ⁶⁶Zn: ~0.36 ‰) and partly weathered carbonate bedrock (δ⁶⁶Zn: ~0.58 ‰), which contain heavier Zn isotopes than the nodules. Synchrotron-based X-ray fluorescence (μ-SXRF) shows that Zn is well correlated with both Fe and Mn. X-ray absorption near edge spectroscopy (XANES) measurements reveal that Zn is associated with both goethite and birnessite phases, with goethite-sorbed Zn accounting for ~76 % of the total Zn and birnessite-sorbed Zn accounting for ~24 %. By combining these new results, the isotopically light Zn in the FeMn nodules compared to their sources can be explained by equilibrium sorption of Zn on goethite and birnessite, during which light Zn is preferentially sorbed. Our study provides important new data on Zn isotope compositions in terrestrial soil FeMn nodules and constrains associated mechanisms, and have implications for using Zn isotopes as environmental tracers.