Precise and differentiated solutions for safe usage of Cd-polluted paddy fields at regional scale in southern China: Technical methods and field validation

Precision management of Cd-contaminated paddy fields with high geochemical backgrounds in karst regions: integrating Bayesian decision tree and spatial zoning

Cadmium (Cd) contamination in paddy fields with high geochemical backgrounds in karst regions poses significant challenges. This study aims to explore practical technologies applicable to different risk causal zones throughout karstic Cd-polluted paddy fields, by integrating a Bayesian decision tree-based risk warning model with spatial zoning. In the study area, 92.1 % of soil and 30.1 % of rice samples exceeded Cd limits, yet the synergy between elevated Cd levels in soil and rice was weak. The model achieved an accuracy of 93.4 % in predicting Cd risk in rice and generated 13 risk classification rules for evaluating Cd risk levels in paddy fields. Notably, the identified risk zones were primarily concentrated in areas with relatively low total Cd concentrations in the soil. Regional management framework, including passivation techniques, phytoremediation, agronomic regulation, was proposed according to 7 causal rules across the fields. Each zone was associated with a customized method for rice safe production, subsequently developing a spatially precise, differentiated solution. This study offers new insights into managing Cd-contaminated paddy fields in karst regions.

Influence of cropping rotation and straw management on Cd accumulation in rice (Oryza sativa L.): a perspective from the rape (Brassica campestris)-rice system

Cadmium (Cd) contamination is a major concern due to its toxicity and bioaccumulation in crops. The study researched the impact of rape (Brassica campestris) cultivation and influence of straw management on Cd accumulation in rice (Oryza sativa L.) under rape-rice rotation system. Field experiments were set in moderately Cd-contaminated farmlands, with treatments including rape-rice rotation with straw removal (NR), rape-rice rotation with straw return (RR), and single cropping rice (SR). In contrast with SR, the NR effectively reduced 6.67-19.59% soil available Cd and the 37.18-39.10% rice grains Cd accumulation during 3 years. Compared with the RR, SR significantly decreased 17.17-27.78% soil available Cd concentrations and 33.30-53.84% rice grain Cd accumulation. Soil physical and chemical properties were influenced by rape cultivation and straw return, further affecting Cd mobility. This study underscores that rape-rice rotation with straw removal can be an effective tactic for reducing the Cd contamination in rice grains and improve soil environment. This research provides critical insights into sustainable agricultural practices for Cd-contaminated soils, offering practical solutions for mitigating Cd risk in rice, while highlighting the importance of straw management in crop rotations.

Sustainable remediation of Cd-contaminated farmland through the rotation of rapeseed-rice varieties with different Cd accumulation potentials

In response to the safety risks posed by cadmium (Cd)-contaminated rice fields worldwide, a suitable production-and-restoration strategy is required for actual agricultural practices. To investigate the remediation effects of different accumulation varieties in rapeseed-rice cropping systems and their influence on Cd migration and transportation, field experiments were conducted based on different planting combinations (FWHR, conventional rice variety (HR) monoculture under fallow; FWLR, low Cd-accumulating rice variety (LR) monoculture under fallow; LOLR, LO (low Cd-accumulating rapeseed variety)-LR rotation; LOHR, LO-HR rotation; HOLR, HO (high Cd-accumulating rapeseed variety)-LR rotation; HOHR, HO-HR rotation). The study found that a rapeseed and rice rotation with appropriate varieties could reduce the rice grain Cd content, increase rice yield, and remove soil Cd without affecting agricultural production efficiency. Compared to the fallow-conventional rice pattern, various rapeseed-rice rotations reduced the Cd content of rice grains by 15 %-38 %, and significantly increased the available potassium (Ava-K) in the subsequent rice soil by 29.6-56.4 mg/kg. The total economic benefits increased by $500-$1800 per hectare. A high accumulation variety of rapeseed and low accumulation variety of rice produced the most effective reduction in Cd levels, with a reduction rate of 38 % in brown rice and an annual removal rate of 24.42 g/hm2. This combination also resulted in a 29 % increase in rice yield compared to the fallow-low accumulation variety rice pattern. Structural equation modeling revealed that with the combined action of crop rotation and variety selection the crop rotation directly reduced the soil available Cd or had an indirect effect by weakening the root-zone acidification effect and increasing soil Ava-P. The rotation of rapeseed and rice with carefully selected matching varieties is a feasible solution for the safe production and pollution remediation of Cd-contaminated paddy fields.

Optimizing Soil Sampling for Accurately Prediction of the Potential Remediation-Effective Area in a Contaminated Agricultural Land

To achieve food security in a contaminated agricultural land, the remediation areas usually need more samples to obtain accurate contamination information and implement appropriate measures. In this study, we propose an optimal encryption sampling design to instead of the detailed survey, which is determined by the variation of heavy metals and the technology capability of remediation, to guide soil sampling for accurately remediation in the potential remediation-effective areas (PRA). The coefficient of screening variation threshold (CSVT), considering spatial variation, technology capacity and acceptable error of sampling, together with the spatial cyclic statistics method of neighbourhood analysis, is introduced to identify and delineate the PRA. Both of the hypothetical analysis and application case studies are conducted to illustrate the advantages and disadvantages of the optimization. The results show that, compared with the detailed survey, the optimal design shows a lower overall accuracy due to its sparsely sampling at the clean area, but it exhibits a similar effect of accurately prediction in boundary delineation and further classification in the PRA in both simulation and application studies. This work provides an effective method for subsequent accurate remediation at the investigation stage and valuable insights into application combination of technology capacity and contaminated agricultural land investigation.

Varietal responses to a soil amendment: Balancing cadmium mitigation and mineral biofortification in wheat production

The application of soil amendment (SA) and the cultivation of low Cd-accumulating varieties have been a widely favored strategy to enable the safe utilization of Cd-contaminated arable land. However, little has been reported on the reciprocal effects of SA on the Cd mitigation and nutritional quality of different wheat varieties. In this study, we evaluated the impact of an SA on agronomic traits, Cd accumulation, translocation and mineral nutrition of 12 wheat varieties in an acidic field with a Cd concentration of 0.46 mg/kg. The results showed that the SA significantly reduced soil DTPA Cd (42.3 %) and resulted in a slight decrease in wheat grain yield (4.24-9.72 %, average 7.62 %). Similarly, the SA significantly reduced grain Cd concentrations (average 61.65 %) while increased the concentrations of beneficial elements such as Mo and Se in all wheat varieties. However, this intervention also led to a reduction in the concentration of essential mineral elements (such as Ca, Fe, and Mn) in whole wheat grain and starchy endosperm, as well as a reduction in their proportion in the bran. Based on genotypic differences, Huaimai 33, Zhenmai 168, Sumai 188 and Yangmai 28 were considered to be the relatively most promising wheat varieties for achieving a balance among food safety, nutritional quality, and economic yield in this region. Taken together, this study highlights the varietal differences in Cd mitigation and mineral accumulation in different wheat varieties in response to the SA, offering new perspectives for phytoremediation and biofortification strategies for Cd-contaminated farmland.

Speciation, leachability, and phytoaccessibility of heavy metals during thermochemical liquefaction of contaminated peanut straw

In this study, the speciation, leachability, phytoaccessibility, and environmental risks of heavy metals (Cd, Zn, and Cu) during liquefaction of contaminated peanut straw in ethanol at different temperatures (220, 260, 300, 340, and 380 °C) were comprehensively investigated. The results showed that elevated temperatures facilitated heavy metal accumulation in the biochar. The acid-soluble/exchangeable and reducible fraction percentages of heavy metals were substantially reduced in the biochar after liquefaction as the temperature increased, and the oxidizable fraction became the dominant heavy metal fraction, accounting for 44.14-78.67%. Furthermore, although an excessively high liquefaction temperature (380 °C) increased the residual fraction percentages of Zn and Cu, it was detrimental to Cd immobilization. The acid-soluble/exchangeable Cd in the contaminated peanut straw readily migrates to the bio-oil during liquefaction, with the highest concentration of 1.60 mg/kg at 260 °C liquefaction temperature, whereas Zn and Cu are predominantly bound to the unexchangeable fraction in the bio-oil. Liquefaction inhibited heavy metal leachability and phytoaccessibility in biochar, the lowest extraction rates of Cd, Zn, and Cu were 0.71%, 1.66% and 0.95% by diethylenetriamine pentaacetic acid, respectively. However, the leaching and extraction concentrations increased when the temperature was raised to 380 °C. Additionally, heavy metal risk was reduced from medium and high risk to no and low risk. In summary, liquefaction reduces heavy metal toxicity and the risks associated with contaminated peanut straw, and a temperature range of 300-340 °C for ethanol liquefaction can be considered optimal for stabilizing heavy metals.

Overexpression of LSU1 and LSU2 confers cadmium tolerance by manipulating sulfur metabolism in Arabidopsis

Phytoremediation using plants is an environmentally friendly and cost-effective strategy for removing cadmium (Cd) from soil. Plants used for phytoremediation must have a high Cd accumulation capacity and strong Cd tolerance. Therefore, understanding the molecular mechanism of Cd tolerance and accumulation in plants is of great interest. In response to Cd exposure, plants produce various thio-rich compounds, such as glutathione, phytochelatins, and metallothioneins, which play important roles in Cd immobilization, sequestration, and detoxification. Therefore, sulfur (S) metabolism is crucial for Cd tolerance and accumulation. In this study, we report that the overexpression of low-S responsive genes, LSU1 and LSU2, confers Cd tolerance in Arabidopsis. First, LSU1 and LSU2 promoted S assimilation under Cd stress. Second, LSU1 and LSU2 inhibited the biosynthesis and promoted the degradation of aliphatic glucosinolates, which could limit the consumption and enhance the release of S, thus, facilitating the production of the S-rich metabolites, glutathione, phytochelatins, and metallothioneins. We further demonstrated that the Cd tolerance mediated by LSU1 and LSU2 was dependent on the myrosinases BGLU28 and BGLU30, which catalyze the degradation of aliphatic glucosinolates. In addition, the overexpression of LSU1 and LSU2 improved Cd accumulation, which has great potential for the phytoremediation of Cd-contaminated soil.

Prediction of cadmium bioavailability in the rice-soil system on a county scale based on the multi-surface speciation model

Relative to total cadmium (Cd) content, bioavailable Cd in paddy soil is regarded as a more reasonable indicator for the risk of Cd bioaccumulation in rice. However, there is still a lack of approach to accurately predict the content of bioavailable Cd in paddy soil due to its heterogeneity and complexity. Here, multi-surface speciation model (MSM) was employed to predict the bioavailable Cd and Cd immobilization effect. Moreover, a precise remediation strategy was designed based on screening and scenario simulation of the sensitive factors with MSM. The results demonstrated that MSM can well predict Cd bioaccumulation risk in rice. The contribution of pH to Cd bioavailability was quantified under three analysis scenarios, accounting for 87.51% of the total variance of bioavailable Cd. In addition, the pH alert value (6.31 ± 0.52) for Cd risk was acquired for each rice field on a county scale. A precise map for the application amount of lime materials was constructed by taking CaCO₃ (3.38-15.75 t ha^-1) as a recommended economical and green immobilization agent. This study provides a potentially effective approach for risk assessment of Cd contamination in rice and important reference for precise Cd remediation in paddy soil.

Migration and transformation of Cd in four crop rotation systems and their potential for remediation of Cd-contaminated farmland in southern China

A crop rotation system combining agricultural production with phytoremediation is an economical and sustainable method of remediation of cadmium (Cd)-contaminated farmland. This study focuses on migration and transformation of Cd in rotation systems and the influencing factors. In a two-year field experiment, four rotation systems were evaluated: traditional rice and oilseed rape (TRO), low-Cd rice and oilseed rape (LRO), maize and oilseed rape (MO), and soybean and oilseed rape (SO). Oilseed rape is a remediation plant in rotation systems. Compared to 2020, the grain Cd concentrations of traditional rice, low-Cd rice, and maize in 2021 decreased by 73.8 %, 65.7 %, and 24.0 % (below the safety limits), respectively. However, soybean increased by 71.4 %. The LRO system featured the highest oil content of rapeseed (about 50 %) and economic output/input ratio (1.34). Removal efficiency of total Cd in soil was 10.03 % (TRO) > 8.3 % (LRO) > 5.32 % (SO) > 3.21 % (MO). Crop uptake of Cd was influenced by bioavailability of soil Cd, and soil environmental factors regulated the bioavailable Cd. Redundancy analysis (RDA) indicated that soil nitrate‑nitrogen (NO₃^--N) had a dominant impact on bioavailable Cd in soil, with variance contributions of 56.7 % for paddy-upland (TRO and LRO) and 53.5 % for dryland (MO and SO) rotation systems. The difference reflected that ammonium N (NH₄⁺-N) was a secondary factor in paddy-upland rotations, while it was the available phosphorus (P) in dryland rotations, with variance contributions of 10.4 % and 24.3 %, respectively. The comprehensive evaluation of crop safety, production, economic benefits, and remediation efficiency revealed that the LRO system was efficient and more acceptable to local farmers, providing a new direction for the utilization and remediation of Cd-contaminated farmland.