Shiqiang Wei

The karst region of Southwest China represents a typical high geological background area characterized by extensive carbonate bedrock and secondary enrichment of heavy metals, particularly cadmium (Cd), in residual soils. Under natural carbonate-buffered conditions, Cd is largely immobilized through mineral associations and surface complexation, resulting in elevated total concentrations but low bioavailability. However, intensified anthropogenic pressures–including acid deposition, mining, excessive fertilization, and improper irrigation—have accelerated soil acidification in paddy fields. Acidification disrupts carbonate geochemical equilibria, weakens buffering capacity, and drives Cd speciation shifts toward more labile forms, thereby enhancing plant uptake and accumulation. These effects are especially pronounced in paddy fields and other systems subject to hydrological and redox fluctuations that further increase Cd mobility. To evaluate these coupled geogenic and anthropogenic controls, we conducted a structured literature synthesis (2016–2026) focusing on peer-reviewed studies of Cd dynamics in Southwestern China’s karst agroecosystems. We critically examine (i) the formation mechanisms and spatial heterogeneity of high-background Cd, (ii) acidification-driven speciation transformation and soil–crop transfer pathways, and (iii) in situ remediation and precision risk assessment strategies. By integrating geological inheritance, geochemical activation, and ecological risk perspectives, this review proposes a conceptual framework to support soil quality standard refinement and adaptive risk management in high-background karst regions.

Soil heavy metal contamination has emerged as a global environmental and public health challenge. Among them, cadmium (Cd) is of particular concern due to its high mobility and ecotoxicity. To identify the key limiting factors and their nonlinear threshold effects for Cd accumulation in maize grains (Grain-Cd) in heterogeneous soil environments, a coordinated sampling campaign of soil and maize was conducted at the municipal scale in Chongqing, China. A total of 499 paired soil–maize samples were obtained, and the correlations between Grain-Cd concentrations and soil physicochemical properties, as well as soil Cd pollution characteristics, were quantitatively evaluated using the integrated Random Forest (RF) model and SHAP (SHapley Additive exPlanations) algorithm instead of traditional linear statistical methods. The results showed that the average Cd content in the soil of maize-growing areas in Chongqing City was 0.30 mg·kg⁻¹, with a variation coefficient (CV) of 53%, and the spatial heterogeneity was significant. The average Cd content in maize grains was 0.03 mg·kg⁻¹, with an exceedance rate of 9.6% over the Chinese National Standard (0.10 mg·kg⁻¹), indicating a certain food safety risk. The RF model achieved a high predictive accuracy for Grain-Cd (R² = 0.815, RMSE = 0.028 mg·kg⁻¹, MAE = 0.013 mg·kg⁻¹), which was significantly superior to the traditional linear regression model (R² = 0.526, RMSE = 0.0459 mg·kg⁻¹). The available Cd (avlCd) in the soil was identified as the core controlling factor for the Grain-Cd content, while total soil Cd (SCd) only showed its positive contribution at contents higher than 0.5 mg·kg⁻¹. Soil pH, CEC (cation exchange capacity), and total phosphorus (TP) exerted significant influences on the Grain-Cd by regulating soil avlCd. The dependence of Grain-Cd on these soil factors was typically nonlinear, and an obvious turning point (threshold) existed for each factor with its occurring level in soil, determined by SHAP analyses as avlCd: 0.29 mg·kg⁻¹, pH: 6.58, CEC: 18.9 cmol (+)/kg, and TP: 0.5 g·kg⁻¹, respectively. This study clarifies the nonlinear regulatory mechanisms of key soil factors on Cd accumulation in maize grains in Chongqing, and the established RF-SHAP framework and identified soil factor thresholds lay a scientific foundation for the interpretable quantification of the soil–maize Cd system, while providing a scientific basis for the precise, targeted remediation of Cd-contaminated dryland farmland and the assurance of regional maize production safety.

Arsenic (As) and cadmium (Cd) in rice grains are major global food safety concerns. Iron (Fe) can help reduce both, but current Fe treatments suffer from poor stability, low leaf absorption, and fast soil immobilization, with unclear underlying mechanisms. To address these issues, an Fe-based metal–organic framework (MIL-88) was modified with sodium alginate (SA) to form MIL-88@SA. Its stability as a foliar inhibitor and its leaf absorption were tested, and its effects on As and Cd accumulation in rice were compared with those of soluble Fe (FeCl₃) and chelating Fe (HA + FeCl₃) in a field study on As–Cd co-contaminated rice paddies. Compared with the control, MIL-88@SA outperformed or matched the other Fe treatments. A single foliar spray during the tillering stage increased the rice yield by 19% and reduced the inorganic As and Cd content in the grains by 22.8% and 67.8%, respectively, while the other Fe treatments required two sprays. Its superior performance was attributed to better leaf affinity and thermal stability. Laser ablation inductively coupled plasma–mass spectrometry (LA–ICP–MS) and confocal laser scanning microscopy (CLSM) analyses revealed that Fe improved photosynthesis and alleviated As–Cd stress in leaves, MIL-88@SA promoted As and Cd redistribution, and Fe–Cd co-accumulation in leaf veins enhanced Cd retention in leaves.

Soil aggregates play critical roles in regulating the behavior of heavy metal in soils. To understand the distribution of cadmium (Cd) in aggregates of different soil types, as well as their roles in regulating the Cd bioavailability of bulk soils, four major arable soils, including acidic, neutral, and calcareous purple soils and calcareous yellow soil (APS, NPS, CPS, and CYS), were sampled from Chongqing, China, for aggregate separation and determination of the total Cd(T-Cd) distribution, fractionation, and extractability in various-sized aggregates. A pot experiment with ryegrass (Lolium perenne L.) was conducted to evaluate the Cd bioavailability in bulk soils as influenced by aggregates. The results show that the composition of soil aggregates varies a lot among soils: lower soil pH tends to increase the proportion of macroaggregates while decreasing that of smaller aggregates. The Cd distribution, HCl-extractability, and active fraction (AF, T-Cd/HCl-Cd) in aggregates are all soil type-dependent, with pH and particle size being the main determining factors; the distribution pattern of Cd concentrated in smaller aggregates is only found for CPS and CYS (pH > 7.5) upon exogenous Cd addition, though the finest aggregates (silt–clay, NPS > CPS > CYS, indicating the fundamental influence of soil pH on Cd availability. Higher AF values over bulk soils, either in silt–clay aggregates or in microaggregates (0.053–0.25 mm), whereas lower AF in macroaggregates (1–2 mm) are found for APS and NPS, which correspond to the relative portions of Ex-Cd and Fe/Mn oxide-bound Cd (Fe/Mn-Cd) in these aggregates. In contrast, less variation of AF values among aggregates is observed for CPS and CYS and for APS/NPS upon Cd addition. Pot experiments demonstrated strong positive correlations between ryegrass Cd uptake and HCl-Cd in silt–clay aggregates and T-Cd in microaggregates, while a negative correlation was observed with T-Cd in macroaggregates. These findings supply new insight into the mechanisms of aggregates in controlling Cd bioavailability in bulk soils and shed light on the development of new strategies for remediating Cd-polluted soils.

Soil aggregates play critical roles in regulating the behavior of heavy metal in soils. To understand the distribution of cadmium (Cd) in aggregates of different soil types as well as their roles in regulating Cd bioavailability of the bulk soils, four major arable soils, including acidic, neutral and calcareous purple soils and calcareous yellow soil (APS, NPS, CPS and CYS), were sampled from Chongqing, China for aggregate separation and determination of total Cd(T-Cd) distribution, fractionation, and extractability in various-sized aggregates. A pot experiment with ryegrass (Lolium perenne L) was conducted to evaluate Cd bioavailability in bulk soils as influenced by aggregates. The results show that the composition of soil aggregates varies a lot among soils, lower soil pH tends to increase the proportion of macroaggregate while decreasing that of smaller aggregates. The Cd distribution, HCl-extractability and active fraction (AF, T-Cd/HCl-Cd) in aggregates are all soil type-dependent, with pH and particle size being the main determining factors, the distribution pattern of Cd concentrated in smaller aggregates is only found for CPS and CYS (pH>7.5) upon exogenous Cd addition, though the finest aggregates (silt-clay, < 0.053 mm) consistently exhibited the highest Cd enrichment for all tested soils. The Cd extractability and AF values in all aggregates show a sequence of APS>NPS>CPS>CYS, indicating the fundamental influence of soil pH on Cd availability. Higher AF values over bulk soils either in silt-clay aggregates or in microaggregates (0.053-0.25 mm) whereas lower AF in macroaggregates(1-2mm) are found for APS and NPS, which which correspond to the relative portions of Ex-Cd and Fe/Mn oxide-bound Cd (Fe/Mn-Cd) in these aggregates. In contrast, less variation of AF values among aggregates are observed for CPS and CYS or APS/NPS upon Cd addition. Pot experiments demonstrated strong positive correlations between ryegrass Cd uptake and HCl-Cd in silt-clay aggregates and T-Cd in microaggregates, while a negative correlation was observed with T-Cd in macroaggregates. These findings have supplied new insight into the mechanisms of aggregates in controlling Cd bioavailability in bulk soils, and shed light on the development of new strategies for remediating Cd-polluted soils.

Soil acidification activates most of the cationic heavy metals in soil and thus enhances their accumulation in crops, posing an accentuated threat to human health, while there is limited knowledge regarding the accumulation of metalloid arsenic (As) in crops, which is influenced by acidification due to its opposite behavior in soil. In this study, the acidification processes of neutral purple soil together with the accompanied changes in soil properties and As fractionation were examined through a column-leaching experiment. Subsequently, growth and As accumulation in pakchoi (Brassica campestris L.) were investigated under various combinations of soil pH and As pollution levels in a pot experiment. This allowed us to elucidate the mechanisms of As accumulation in pakchoi under the co-stresses of soil acidification and As pollution. The results indicated that soil acidification followed a two-phase process, initially rapid and later slow, with a turning point at a pH of 4.7–4.8. Below this critical pH, the leaching rates of base ions and As accelerated significantly and the decomposition of primary minerals began, primarily from chlorite to green/mesospheric minerals, resulting in a substantial increase in the content of amorphous iron oxide. Meantime, soil As was transformed from highly labile forms, such as non-specifically and specifically adsorbed forms, to less active ones like amorphous hydrous oxide-bound and residual forms, resulting in decreased As availability. In this context, As pollution remarkably delayed the growth of pakchoi, while the influence of acidification on growth only occurred when the soil was acidified to a pH lower than 6, as demonstrated by a substantial biomass reduction at higher As levels and a 41.8% biomass decrease at pH 4.6. Moreover, soil acidification exacerbated the inhibitory effect of As on pakchoi growth. The As contents in the edible parts of pakchoi dramatically increased with the increase in the soil As level, and soil acidification did not mitigate As accumulation in plants via the suppression of soil As availability but rather greatly increased it due to the bioconcentration effect caused by As toxicity. In conclusion, significant interactions existed between soil acidification and As pollution in terms of soil properties and As transformation, leading to comprehensive effects on growth and As accumulation in crops.

Cadmium (Cd) is a highly toxic heavy metal that is widely distributed in soils at low concentrations. Its volatilization loss occurs at an ashing temperature higher than 350 °C. Accordingly, the accurate determination of its concentration in soils often requires the addition of chemical modifiers by graphite furnace atomic absorption spectrometry (GFAAS) to improve the thermal stability of Cd. In this work, a metal–organic framework (MOF, UIO-66-NH ₂ ) was utilized as a matrix modifier to improve accuracy in determining Cd in soils using GFAAS. The Cd signal, as influenced by drying and pyrolysis temperature and special gas, was also investigated, and the measuring conditions were optimized (2 μg L ⁻¹ Cd concentrations, matrix modifier: UIO-66-NH ₂ , pyrolysis temperature: 550 °C, atomization temperature: 1650 °C, and special gas: air). The limit of detection (LOD) and the characteristic mass for Cd in soil were 3.84 ng g ⁻¹ and 0.80 pg, respectively. Five soil standard reference materials (SRMs) were analyzed for Cd to verify the effectiveness of the optimized measuring conditions. The soil Cd concentrations obtained were in remarkable agreement with the reference values. Moreover, the mechanism of UIO-66-NH ₂ on the graphite platform was investigated in the drying and pyrolysis steps by scanning electron microscopy (SEM), Fourier transform infrared (FT-IR), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). During the drying steps, many functional groups of the matrix were activated, and Cd ²⁺ could be captured on its surface. After the pyrolysis step, the structure matrix collapsed and was partially decomposed into ZrO ₂ , and subsequently, an intermediate state of ZrO ₂ -(CdO) appeared at high temperatures, which came from the active binding sites on the zirconium oxide octahedron nodes.