Zn isotope fractionation in laterites from Yunnan province, southwest China: Implications for the Zn cycles and its environmental impacts in (sub-) tropics

Isotopic signature for the fate of copper in laterites from Yunnan Province, southwest China

Laterites, prevalent in (sub-)tropical regions, form under fluctuating climatic conditions and host redox-sensitive heavy metals like copper (Cu). However, the geochemical behavior of Cu in laterites remains underexplored. To address this issue, this study examined Cu isotope fractionation in laterites from Yunnan Province, southwest China, using inductively multi-collector coupled plasma mass spectrometry (MC-ICP-MS). Cu concentrations ranged from 5.32-34.83 mg/kg in S1 and 4.76-26.7 mg/kg in S2, with δ65Cu values of 0.22-0.66 ‰ (S1) and -1.20-0.33 ‰ (S2). In S1, higher Cu concentrations and heavier δ65Cu suggest retention by Fe oxides under oxidative conditions, potentially influenced by organic matter. In S2, lighter δ65Cu reflects leaching of heavier isotopes at lower pH of laterites (4.5-5.5). Chemical weathering (CIA: 82.65-87.53) promotes aluminization, enhancing Cu mobility in S2, while Fe oxides enrich heavier 65Cu in S1. Cu enrichment in S1, contrasted with depletion in S2, highlights varying contamination risks. These findings highlight redox condition and leaching as key controls on Cu fractionation, offering insights for managing soil contamination in (sub-)tropical regions.

Effects of mulch films with different thicknesses on the microbial community of tobacco rhizosphere soil in Yunnan laterite

The mulch film (MF) management model of the agricultural field affects the physical and chemical properties of soil (PCPS) and the structure of the microorganism community; however, studies on the relationship between the rhizosphere microorganism community structure and the thickness of MF are still limited. To understand the interactions among the MF thickness, PCPS, and rhizosphere microorganism, a study was conducted by using an integrated metagenomic strategy, where tobacco rhizosphere soil was treated with four commonly representative and used thicknesses of MFs (0.004, 0.006, 0.008, and 0.010 mm) in Yunnan laterite. The results showed that agronomic traits such as the tobacco plant height (TPH), leaf number (LN), fresh leaf weight (FLW), and dry leaf weight (DLW) were significantly (p < 0.01) improved in the field mulched with the thickest film (0.010 mm) compared with the exposed field (CK), and there was a 6.81 and 5.54% increase in the FLW and TPH, separately. The correlation analyses revealed a significant positive correlation of the MF thickness with the soil water content (SWC), soil organic matter (SOM), total nitrogen (TN), available nitrogen (AN), total phosphorus (TP), and available phosphorus (AP; all p < 0.01), while the MF thickness was negatively correlated with the soil temperature (ST; p < 0.01). In addition, the community structure of the rhizosphere soil bacteria was significantly changed overall by the MF thickness, which also interfered with the function of the rhizosphere soil bacteria. The correlation analyses also showed that the abundance of Bradyrhizobium and Nitrospira was positively correlated with the MF thickness, while the abundance of Sphinsinomonas and Massilia was negatively correlated with it. This indicated that with the increase of the MF thickness, the ability of the rhizosphere soil to utilize N and remove harmful molecules was strengthened, while the capacity of the rhizosphere soil to degrade pollutants was greatly reduced. These findings provide additional insights into the potential risks of the application of different thicknesses of MFs, particularly concerning the PCPS and soil microbial communities.

Exploration of natural processes and anthropogenic inputs by Zn isotopes in suspended particulate matter: A case study from the Lancang River in Southwest China

River is an important pathway for the biogeochemical cycle of Zn. This study reports Zn concentration and δ66Zn composition for suspended particulate matter (SPM) from Lancang River basin in Southwest China, and explore the impact of natural processes and human activities on Zn cycle. The SPM samples have a much higher average Zn content (162 mg kg-1) than that of the upper crust (67.0 mg kg-1), but it is close to the value of the Pearl River (187 mg kg-1). The enrichment factor (EF) values of Zn in SPM range from 1.08 to 6.88, with an average of 2.15, which does not show significant pollution characteristics. The δ66Zn values in SPM range from -0.67‰ to +0.63‰, with an average of +0.13‰. The δ66Zn values showed positive correlation with Ca/Mg ratios while showed little correlation with Zn contents in SPM. It indicated that anthropogenic sources have limited influence on SPM, and the Zn isotope composition in SPM is more likely to be inherited from the weathered rocks materials and influenced by natural fractionation processes in river water. This result contributes to understanding of the geochemical cycling process of Zn and its environmental effects in water.

Environmental implications of land use change on the fate of Zn in agricultural soils: A case study of Puding karst soils, southwest China

Land use change threatens food security because it may cause the depletion and/or low bioavailability of micronutrients in agricultural soils. Therefore, it is significant to investigate the fate of micronutrients and predict the potential environmental problems. The zinc isotope technique is of particular interest in interpreting soil processes. In this study, Zn isotopic data of soil samples in five profiles based on different land uses were provided, and Zn behavior in different soils was discussed. The isotopic ratios of soil samples in the abandoned orchard, secondary forest, abandoned cropland, and cropland are similar, with the δ⁶⁶Zn varying from 0.15 to 0.29‰. However, the samples in shrub grassland show a lower δ⁶⁶Zn of 0-0.20‰, which may be affected by anthropogenic sources. For the vertical patterns, the non-cultivated long-rooted plants (i.e., abandoned orchard and secondary forest) show no significant difference in the distribution of δ⁶⁶Zn, but the patterns of cropland and abandoned cropland samples are reversed. The cropland samples show positive correlations between δ⁶⁶Zn and Fe₂O₃ (R² = 0.90) and MnO (R² = 0.75), indicating that Fe and Mn oxides preferentially adsorb heavy Zn isotopes on the mineral surfaces. The high affinity between Zn and oxides indicated that the concentration of bio-available Zn in cropland soils was getting lower. As a result, the supplies of micronutrients may be deficient and urged from fertilizer. This study provides a better understanding of Zn cycling in agricultural systems and gives improvements in soil management.