Cd Accumulation In Soil Research Articles

Prediction of cadmium content using machine learning methods

Heavy metals are the most environmentally hazardous pollutions in agricultural soils, threatening humans and several ecosystem services. Cadmium (Cd) is a highly toxic element but distinctively different from other heavy metals with its high mobility in soil environments. The study aimed to evaluate the Cd concentration of soils in the Konya plain with a specific attribute to soil fertilization, mainly phosphorous fertilizers. A total of 538 surface (0–20 cm) soil samples were analyzed to determine basic physical and chemical properties and total phosphorus (P) and Cd concentrations. Descriptive statistics, machine learning, and regression models were used to assess the accumulation of Cd in soils. Decision Trees, Linear Regression, Random Forest, and XGBoost machine learning methods were used in Cd prediction. The XGBoost model proved to be the best prediction model, with a coefficient of determination of 98.1%. Electrical conductivity, pH, CaCO3, silt, and P were used in the Cd estimation of the XGBoost model and explained 56.51% of the total variance in relation to measured soil properties. The results revealed that a machine learning algorithm could be useful for estimating Cd concentration in soils using basic physical and chemical soil properties.

Effects of polyurethane microplastics combined with cadmium on maize growth and cadmium accumulation under different long-term fertilisation histories

Agricultural production uses different types of fertilisation treatments, typically employing the combined application of organic fertiliser (OF) or organic–inorganic fertiliser (OIF) to improve soil quality. When coupled with cadmium (Cd), microplastics (MPs) affect plant growth and Cd accumulation in soils treated with different fertilisers. This study systematically examined the effects of polyurethane (PU) MPs coupled with Cd on the growth characteristics, root metabolite characteristics, rhizosphere bacterial community structure, and Cd bioavailability of maize under different long-term fertilisation treatments and soil types (red/cinnamon soil). The combined effects of PU MPs and Cd on maize growth differed across fertilisation treatments. Under OF, maize plants accumulated more Cd than under OIF. The accumulation of Cd in maize plants in red soil was twice that in cinnamon soil. Under OF, PU MPs promoted Cd activation by decreasing the soil pH, while root metabolites promoted Cd adsorption sites by synthesising specific amino acids, degrading aromatic compounds, and synthesising pantothenic acid and coenzyme A. Under OF, PU MPs can lower the soil pH to promote the activation of cadmium, while root metabolites promote root growth and increase cadmium adsorption sites by synthesizing specific amino acids, degrading aromatic compounds, and synthesizing pantothenic acid and coenzyme A, hereby promoting root Cd absorption. Under OIF, PU MPs act by influencing the biosynthesis of amino acids in root metabolites, enriching energy metabolism pathways, promoting the transport and translocation of mineral nutrients, thereby amplifying the "toxic effects" of Cd. This study provides new insights into the risk assessment of PU MPs and Cd coupling under different fertilisation treatments, and suggests that the prevention and control of combined PU MPs and Cd pollution in red soil under OF treatment should receive more attention in the future.

Development and application of machine learning models for prediction of soil available cadmium based on soil properties and climate features

Identifying the key influencing factors in soil available cadmium (Cd) is crucial for preventing the Cd accumulation in the food chain. However, current experimental methods and traditional prediction models for assessing available Cd are time-consuming and ineffective. In this study, machine learning (ML) models were developed to investigate the intricate interactions among soil properties, climate features, and available Cd, aiming to identify the key influencing factors. The optimal model was obtained through a combination of stratified sampling, Bayesian optimization, and 10-fold cross-validation. It was further explained through the utilization of permutation feature importance, 2D partial dependence plot, and 3D interaction plot. The findings revealed that pH, surface pressure, sensible heat net flux and organic matter content significantly influenced the Cd accumulation in the soil. By utilizing historical soil surveys and climate change data from China, this study predicted the spatial distribution trend of available Cd in the Chinese region, highlighting the primary areas with heightened Cd activity. These areas were primarily located in the eastern, southern, central, and northeastern China. This study introduces a novel methodology for comprehending the process of available Cd accumulation in soil. Furthermore, it provides recommendations and directions for the remediation and control of soil Cd pollution.

Effect of intercropping on the Cd accumulation in soil and rice plants: A review

Effect of intercropping on the Cd accumulation in soil and rice plants: A review

Partial Substitution of Chemical N with Solid Cow Manure Improved Soil Ecological Indicators and Crop Yield in a Wheat–Rice Rotation System

Alternative fertilizers are essential to minimizing the deteriorating effects of chemical fertilizers on soil and water quality/health. Accordingly, the present work investigated the effects of combined organic–inorganic fertilization (COIF) on wheat and rice yields, soil nutrients, and soil Cd accumulation. Hence, seven different treatments were set up: control (CK); conventional fertilization (CF); adequate fertilization (OF); organic fertilizer replacing 25% (T1) and 50% (T2) of OF; and organic nitrogen (N) replacing 25% (M1) and 50% (M2) of OF-N. Overall, significant increases occurred in the yields of COIF crops. Compared with the CF, the highest wheat and rice yields happened in the M1 treatment (with a difference of approximately 18.5%) (p < 0.05). COIF slightly alleviated soil acidification, and improved the cation exchange capacity (CEC) of the study soils. Furthermore, COIF treatments significantly increased the contents of total phosphorus, total potassium, available phosphorus, and available potassium by 6.35 to 16.9%, 3.17 to 10.9%, 5.53 to 28.7%, and 2.6 to 12%, respectively (p < 0.05). Nevertheless, negligible increases took place in the Cd content of COIF soils compared with that of the CK. Altogether, our results concluded that 25% replacement of OF-N by organic N (M1) effectively improved the fertility/ecological sustainability of the study soils.

Biochar and organic fertilizer drive the bacterial community to improve the productivity and quality of Sophora tonkinensis in cadmium-contaminated soil.

Excessive Cd accumulation in soil reduces the production of numerous plants, such as Sophora tonkinensis Gagnep., which is an important and widely cultivated medicinal plant whose roots and rhizomes are used in traditional Chinese medicine. Applying a mixture of biochar and organic fertilizers improved the overall health of the Cd-contaminated soil and increased the yield and quality of Sophora. However, the underlying mechanism between this mixed fertilization and the improvement of the yield and quality of Sophora remains uncovered. This study investigated the effect of biochar and organic fertilizer application (BO, biochar to organic fertilizer ratio of 1:2) on the growth of Sophora cultivated in Cd-contaminated soil. BO significantly reduced the total Cd content (TCd) in the Sophora rhizosphere soil and increased the soil water content, overall soil nutrient levels, and enzyme activities in the soil. Additionally, the α diversity of the soil bacterial community had been significantly improved after BO treatment. Soil pH, total Cd content, total carbon content, and dissolved organic carbon were the main reasons for the fluctuation of the bacterial dominant species. Further investigation demonstrated that the abundance of variable microorganisms, including Acidobacteria, Proteobacteria, Bacteroidetes, Firmicutes, Chloroflexi, Gemmatimonadetes, Patescibacteria, Armatimonadetes, Subgroups_ 6, Bacillus and Bacillus_ Acidiceler, was also significantly changed in Cd-contaminated soil. All these alterations could contribute to the reduction of the Cd content and, thus, the increase of the biomass and the content of the main secondary metabolites (matrine and oxymatrine) in Sophora. Our research demonstrated that the co-application of biochar and organic fertilizer has the potential to enhance soil health and increase the productivity and quality of plants by regulating the microorganisms in Cd-contaminated soil.

Model development and probabilistic risks of cadmium transport in slag-soil-groundwater systems with heterogeneous conditions

Prediction of the long-term risk of trace metals leaching from soils at smelting sites is essential for groundwater protection. Herein, a mass balance-based stochastic model was developed to simulate the transport and probabilistic risks of trace metals in heterogeneous slag-soil-groundwater systems. The model was applied to a smelting slag yard with three stacking scenarios, including (A) fixed stacking amount, (B) stacking amount increasing yearly, and (C) slag removal after 20 years. The simulations suggested that the leaching flux and net accumulation of Cd in soils of the slag yard and abandoned farmland were greatest for scenario (B), which was followed by scenarios (A) and (C). In the slag yard, a plateau occurred in the Cd leaching flux curves, followed by a sharp increase. After 100 years of leaching, only scenario (B) had a high probabilistic risk (>99.9 %) of threatening groundwater safety under heterogeneous conditions. <11.1 % of the exogenous Cd may leach into groundwater under the worst scenario. The key parameters affecting Cd leaching risk include runoff interception rate (IRCR), input flux from slag release (I), and stacking time (ST). The simulation results were consistent with the values measured in a field investigation and laboratory leaching experiments. The results should help guide remediation objectives and measures to minimize the leaching risk at smelting sites.

Straw removal or non-removal affects cadmium (Cd) accumulation in soil–rice (Oryza sativa L.) system at different ambient air Cd levels

Despite the potential importance of the removal of contaminated straw for heavy metal output from agricultural soils, previous studies have primarily focused on the variation in the metal concentration without considering the impact input of heavy metals from atmospheric deposition. Here, rice was grown under field conditions, and, as a reference, in a deposition-free environment, and exposed to different ambient air Cd levels. Two consecutive years of pot experiments were conducted in two study areas (ZZ and LY) to examine the changes in soil physicochemical properties as well as Cd accumulation in the soil–rice (Oryza sativa L.) system in response to straw return or removal. The results showed that rice straw return enhanced the soil pH and organic matter (OM) content, but reduced the soil redox potential (Eh); and the variation in amplitude increased with number of cultivation years. After two years of cultivation, the concentrations of soil total Cd and extractable Cd in the straw-removal treatments reduced by 9.89–29.49% and 4.88–37.74%, respectively, whereas those in the straw-return treatments exhibited a slight decrease, or even an increase. This indicated that straw removal could effectively reduce the concentration and bioavailability of Cd in contaminated farmland, which was further confirmed by the results for accumulation of Cd in rice tissues. In addition, the contribution from atmospheric deposition was confirmed by the greater variation in Cd concentration in soils and rice tissues under deposition-free conditions. A major implication of our findings is that the adoption of reasonable straw-treatment measures and proper control over ambient air heavy metals can promote the remediation efficiency of Cd-contaminated fields.

Linking atmospheric emission and deposition to accumulation of soil cadmium in the Middle-Lower Yangtze Plain, China

Cadmium (Cd) is one of the most toxic heavy metals in the environment. Atmospheric deposition has been found to be the main source of Cd pollution of soil on a large scale in China, and identification of the relationships between anthropogenic emission, atmospheric deposition, and Cd accumulation in soil is important for developing ways to mitigate Cd non-point pollution. In this study, the relationship between atmospheric emission, atmospheric deposition, and soil Cd accumulation in the Middle-Lower Yangtze Plain in China was investigated using datasets of atmospheric emission, deposition, and soil accumulation from the literatures published between 2000 and 2020. The results showed that the soil Cd accumulation rate in the study area exceeded the national average (4.0 μg kg–1 yr–1) and continued to accumulate in recent decades, although the average accumulation rate decreased from 9.45 μg kg–1 yr–1 (2000–2010 period) to 8.86 μg kg–1 yr–1 (2010–2020 period). The contribution of atmospheric deposition flux to Cd increment in the soil was in the range of 22–29%, with the atmospheric deposition flux decreasing from 0.54 mg m–2 yr–1 (2000–2010) to 0.48 mg m–2 yr–1 (2010–2020), both values being greater than the national average. Atmospheric Cd deposition and emission were highly correlated in a provincial administrative region, which is close to a ratio of 1.0. Emission factors may be in a state of dynamic change due to the influences of new Cd emission control technologies and environmental policies. As the main sources of Cd emissions, dust, and smoke emissions per ton of non-ferrous metal production decreased by 64.7% between the 2000–2010 and 2010–2020 periods. Although new environmental policies have been instigated, atmospheric emission of Cd is still excessive. It was hoped that the findings of this work would provide a scientific basis for the rational control of atmospheric emissions and Cd pollution of soil.

Annual net input fluxes of cadmium in paddy soils in karst and non-karst areas of Guangxi, China

In recent years, the problems of high cadmium content and low Cd mobility in karst areas compared with non-karst areas have attracted extensive attention. Identifying the difference between the net Cd flux in karst and non-karst areas is crucial for sustainable land management. Basically, rock weathering is a natural source of Cd. While Cd accumulation in soil has increased due to both anthropogenic and natural sources, the contributions of anthropogenic input and rock weathering to soil Cd concentration in karst and non-karst areas remain unclear. In this study, the input (e.g., atmospheric deposition, fertilizer and irrigation) and output fluxes (e.g., leaching and early and later rice harvest) of Cd in farmland soils in karst and non-karst areas of Guangxi from 2017 to 2018 were monitored. The results showed no significant difference (P = 0.289) in the contributions of various input pathways between karst area and non-karst area, and atmospheric deposition (64–55 %) played an important role in the input fluxes. Nevertheless, the low soil pH value and high rainfall in non-karst areas increase Cd mobility and plant availability in soil, causing the output flux in non-karst areas (5.30 g/ha/y) to be greater than that in karst areas (3.17 g/ha/y). According to the national standard, the exceedance rate of Cd in rice grains in non-karst area (17 %) was higher than that in karst area (15 %). Therefore, ecological risk assessment in non-karst areas with high Cd bioavailability should be prioritized. Furthermore, the annual increase rate of Cd concentration in topsoil of karst and non-karst areas was −0.27 μg/kg/y and −1.23 μg/kg/y, respectively. However, the soil Cd concentration in karst and some non-karst areas still exceeded the guideline value. Given that the net input flux of Cd was negative, the accumulation of Cd in soil may be determined by rock weathering.

Source-specific risk assessment for cadmium in wheat and maize: Towards an enrichment model for China

Cadmium (Cd) pollution of agricultural soil is of public concern due to its high potential toxicity and mobility. This study aimed to reveal the risk of Cd accumulation in soil and wheat/maize systems, with a specific focus on the source-specific ecological risk, human health risk and Cd enrichment model. For this we investigated more than 6100 paired soil and grain samples with 216 datasets including soil Cd contents, soil pH and grain Cd contents of 85 sites from China. The results showed that mining activities, sewage irrigation, industrial activities and agricultural practices were the critical factors causing Cd accumulation in wheat and maize cultivated sites. Thereinto, mining activities contributed to a higher Cd accumulation risk in the southwest China and Middle Yellow River regions; sewage irrigation influenced the Cd accumulation in the North China Plain. In addition, the investigated sites were classified into different categories by comparing their soil and grain Cd contents with the Chinese soil screening values and food safety values, respectively. Cd enrichment models were developed to predict the Cd levels in wheat and maize grains. The results showed that the models exhibited a good performance for predicting the grain Cd contents among safe and warning sites of wheat (R2 = 0.61 and 0.72, respectively); while the well-fitted model for maize was prone to the overestimated sites (R2 = 0.77). This study will provide national viewpoints for the risk assessments and prediction of Cd accumulation in soil and wheat/maize systems.

Rhizobacteria communities reshaped by red mud based passivators is vital for reducing soil Cd accumulation in edible amaranth

Red mud (RM) was constantly reported to immobilize soil cadmium (Cd) and reduce Cd uptake by crops, but few studies investigated whether and how RM influenced rhizobacteria communities, which was a vital factor determining Cd bioavailability and plant growth. To address this concern, high-throughput sequencing and bioinformatics were used to analyze microbiological mechanisms underlying RM application reducing Cd accumulation in edible amaranth. Based on multiple statistical models (Detrended correspondence analysis, Bray-Curtis, weighted UniFrac, and Phylogenetic tree), this study found that RM reduced Cd content in plants not only through increasing rhizosphere soil pH, but by reshaping rhizobacteria communities. Special taxa (Alphaproteobacteria, Gammaproteobacteria, Actinobacteriota, and Gemmatimonadota) associated with growth promotion, anti-disease ability, and Cd resistance of plants preferentially colonized in the rhizosphere. Moreover, RM distinctly facilitated soil microbes' proliferation and microbial biofilm formation by up-regulating intracellular organic metabolism pathways and down-regulating cell motility metabolic pathways, and these microbial metabolites/microbial biofilm (e.g., organic acid, carbohydrates, proteins, S2−, and PO43−) and microbial cells immobilized rhizosphere soil Cd via the biosorption and chemical chelation. This study revealed an important role of reshaped rhizobacteria communities acting in reducing Cd content in plants after RM application.

Assessment of Cd bioavailability using chemical extraction methods, DGT, and biological indicators in soils with different aging times

Total cadmium (Cd) cannot be used to accurately assess the ecological risk of Cd pollution in soil. Currently there is no universally recognized method to evaluate Cd bioavailability in soil. In this study, chemical extraction methods, diffusive gradients in thin films (DGT) and bioindicator methods were used to evaluate Cd bioavailability in soils with the same properties but different aging times. Results indicate that aging decreased the Cd bioavailability in soil and its toxicity to barley. This was primarily due to a decrease in the proportion of ion-exchangeable Cd. Correlation analyses were conducted on the Cd bioavailable content obtained via the soil extraction methods and the toxicity effect of barley. Results showed that the order of the minimum value of the linear regression determination coefficient (R2) of chemical extraction methods and DGT was as follows: DGT-Cd (0.7385, p < 0.05) > total Cd (0.6931, p < 0.05) > acetic acid-Cd (0.6078) > ion-exchangeable Cd (0.5933) > DTPA-Cd (0.5842) > CaCl2-Cd (0.4980) > water-soluble Cd (0.4602). The order of minimum value of R2 of biological indicators of barley was integrated biomarker response (IBR) (0.8501, p < 0.01) > length (0.6492) > dry weight (0.6320) > fresh weight (0.4980) > Cd concentration (0.4602). The root is more suitable for indicating the plant uptake and accumulation of Cd in soil. Meanwhile, the shoot can effectively evaluate the toxic effect of Cd stress on plants. DGT is more suitable to reflect Cd bioavailability to barley compared to chemical extraction methods, Furthermore, it can be used to evaluate stable polluted soil with longer aging time. In the study of the bioavailability of heavy metals in soil, IBR can be used as a reliable reference index to contribute to the comprehensive evaluation of metal bioavailability in addition to considering plant uptake.

The potential role of plasma membrane proteins in response to Zn stress in rice roots based on iTRAQ and PRM under low Cd condition

Cd pollution had already caused serious threats to crop growth and development, food safety and human health, and become a potential agricultural and global environmental problem. Zn had been used to reduce Cd accumulation in soil and plants. Proteins located in plasma membrane (PM) played important roles in transferring stress signals in plants. To further elucidate how PM proteins modulated Zn/Cd transport under low-Cd condition, quantitative proteomics was employed to identify and verify the differentially expressed proteins (DEPs) and their biological functions at proteome level. A total of 4008 proteins were identified, and 332 DEPs (192 up and 140 down, fold >1.50 or <0.66, p < 0.01) were screened. Functional analysis showed that DEPs were mainly catalytic active and binding proteins, involved in glutathione metabolism, phenylpropanoid biosynthesis, etc. DEPs involved in ion transport played key roles in regulating transmembrane transport, resisting stress and alleviating toxicity of heavy metals to rice roots. DEPs were as the marker proteins in rice root responding to heavy metal stress. This study had important guiding significances for metal ions transport mechanism and screening of biomarkers responding to abiotic stress, and provided references for further researches underlying abiotic stress and detoxication in rice and other plants.

Arsenic and Cadmium Accumulation in Soil as Affected by Continuous Organic Fertilizer Application: Implications for Clean Production

As and Cd in soil can be assimilated and accumulated by vegetables and can be subsequently ingested by humans. Contradictory effects of organic fertilizer application on As and Cd accumulation in soil have been reported in previous studies. An eight-year greenhouse study was conducted on a sandy loam soil in Beijing, China to investigate the effects of organic fertilizer application rate on soil properties, and As and Cd accumulation in soil. The contamination risk of pak choi grown after eight years’ application of organic fertilizer was also evaluated. Soil organic carbon increased 3.0–3.8 times with low, medium and high rates of fertilizer application in 2018 compared to the initial soil. Organic fertilizer application significantly increased soil nutrients and microbial biomass while it mildly affected soil pH. The bioavailability of As/Cd has decreased after eight years’ application of organic fertilizer. Pak choi crop harvested from all three treatments in 2018 did not pose a threat to human health, even for life-time consumption. Soil total As content significantly decreased with organic fertilizer application, mainly due to the lower As content in the applied fertilizer than that in soil. Continuous application of clean organic fertilizer can be adopted to reduce the contamination risk of highly contaminated soil in the soil–plant system.

Do soil cadmium concentrations decline after phosphate fertiliser application is stopped: A comparison of long-term pasture trials in New Zealand?

Decreasing soil cadmium (Cd) is one method of removing Cd from the food chain. Phosphorus (P) fertilisers are a major source of Cd inputs into soil. Stopping P fertiliser should theoretically decrease Cd inputs and soil Cd accumulation, but there are few field data to show if this occurs. We examined three long-term grazed pasture trials in New Zealand (Ballantrae, Winchmore and Whatawhata) where P fertiliser had been applied (from 10 to 100 kg P ha−1 yr−1) for up to six years and then stopped for 10 to 26 years. Stopping P fertiliser applications reduced soil Cd concentrations at Winchmore and Whatawhata where P had been applied at ≥34 kg P ha−1 yr−1. No reductions occurred below this rate nor at Ballantrae where only 10 years post P-application data were available. Decreases were ascribed to moderate rainfall (1630 mm at Whatawhata and 740 mm rainfall plus 770 mm irrigation at Winchmore) that enhanced Cd leaching and may have been aided at Winchmore by a decrease in soil pH over time (0.4 units). However, because stopping P fertiliser inputs may quickly impair pasture production, additional strategies may be required to decrease soil Cd quickly.

Cadmium accumulation in rice straws and derived biochars as affected by metal exposure, soil types and rice genotypes

Straw residues, as one of the conservation farming practices, are being strongly encouraged in China, including some cadmium-polluted areas. Nowadays, a portion of this plant residue is promoted to be removed for reducing excess metal(loid) in the soil and to be used for bioenergy production. Nevertheless, the possible influences of contaminated straw or the burial of its derived biochars on Cd accumulation in soil and data based on health risk assessment associated with different status and extent of soil contamination were relatively unknown. Thus it is important to provide a more systematical understanding of contaminated straw burial at specific contamination zones, which may provide useful guidance for straw utilization. In this study, we harvested two genotypes of rice straw from 6 contaminated levels among three soil types to comprehensively study the total Cd contents in straws and its derived biochars and correlate the sets of straw characteristics and Cd contents in three different contamination zones. The total Cd concentration in straws grew at a steady rate relatively with increasing soil Cd contamination levels, compared to those in biochars which performed more fluctuate due to the strong burning. According to correlational analyses, three-way ANOVA showed that the moisture, ash, volatile and fixed carbon were all significantly affected by straw CdTotal contents (p < 0.001). Such relationships were attributed to guide straw removal portions for gasification. Meanwhile, there was a significant correlation between straw Cd concentration and soil types (p < 0.001), confirming that it might be worth determining soil remediation by straw removal according to site-specific farmland conditions. This work will help to assess efforts toward predicting Cd concentration in the paddy soils related to kinds of contamination status and would also give useful guidance to make sustainable management strategies for crop straws in polluted regions. Novelty statement This work provided data on how much rice straw is needed to remove to ensure the minimal amount to control soil contamination and reduce costs according to site-specific conditions and soil Cd contamination status. It also explains the correlations between straw characteristics related to bioenergy use and soil conditions which would give guidance to balance using crop straw for increased bioenergy production and the need to also protect, preserve, and enhance soil resources.

The effects of different electrode materials on seed germination of Solanum nigrum L. and its Cd accumulation in soil

The effects of different electrode on Solanum nigrum L. seed germination were determined. The result showed that germination percentage (GP) of seeds in treatment T2 (titanium electrode) was 26.6% higher than in control (CK, without electric field). High potassium and calcium concentrations were beneficial for seed enzymatic activity in treatment T2, which could partly explain the increase in GP. Cd accumulation (μg/pot) in S. nigrum treated with any electric field was significantly higher (p<0.05) than in CK without electric field. Specifically, Cd accumulation under the treatment T3 (stainless steel electrode) was the highest both in roots and shoots; this accumulation in shoots and roots were 74.7 % and 67.4 % higher for stainless steel than in CK. This increase must have been associated with a higher Cd concentration in plants and did not exert a significant effect on the biomass. In particular, Cd concentrations in roots and shoots under stainless steel treatment were both significantly higher than in CK (p<0.05), which had to be related to the higher available Cd concentration in the soil in the middle region. Furthermore, it could be attributed to altered soil pH and other soil properties. Moreover, none of the biomasses were significantly affected (p<0.05) by different electrode materials compared to CK.

Effectiveness of Water Management in Producing Cadmium-Safe Rice in Soils of Different Contamination Levels and Physicochemical Properties

The effectiveness of water management in reducing cadmium (Cd) accumulation in rice grain has mainly been investigated in slightly and moderately contaminated soils but seldom in heavily contaminated soils. In addition, how soil physicochemical properties influence the effectiveness of water management merits investigation. In this study, a pot experiment was performed using six soils of slight (S1), moderate (S2–S5), and heavy (S6) Cd contamination levels and of different properties. Three water managements, flooding until full tillering (W1), flooding until full tillering and reflooding after 7–10-day drying (W2), and continuous flooding without drying (W3), were implemented for each soil. Cadmium content in rice grain was significantly decreased (by 60% on average) in W2 and further decreased (by 82% on average) in W3 as compared with W1 in all the six soils. Grain Cd contents in the W3 treatments were decreased to below the Chinese food safety standard of 0.2 mg kg−1 in all the six soils except in S6 (0.36 mg kg−1). For all the three water management regimes, the lowest grain Cd content was observed in S3 which had high contents of organic matter, sulfur, and iron. The results indicate that extended flooding can effectively decrease rice grain Cd accumulation in soils of various contamination levels especially those rich in organic matter, sulfur, and iron. But for heavily contaminated soils, water management may need to combine with other approaches to further decrease grain Cd content to meet food safety standard.

Activated low-grade phosphate rocks for simultaneously reducing the phosphorus loss and cadmium uptake by rice in paddy soil.

Cadmium (Cd) pollution and phosphorus (P) leaching in paddy soils has raised the global concern. In this study, two kinds of the low grade phosphate rocks activated by the sodium lignosulfonate (SL) and humic acid (HA) were fabricated for soil Cd passivation and reduction of the soil P leaching simultaneously. The mechanisms of the Cd adsorption and passivation by the activated phosphate rocks (APRs) were investigated through the batch experiment and the indoor culture test (i.e., incubation and pot experiments) in the Cd-polluted paddy soil. The effects of the APRs on the potted rice growth, uptake of Cd by rice and P loss were also studied. In comparison with the superphosphate treatment, the cumulative P loss from SL- and HA-APRs were reduced by the 65.2% and 65.3%. In terms of the Cd passivation, the Cd adsorbed on the APRs was through the chemical ways (i.e., ligand exchange and the formation of internal complexes). The application of the APRs significantly decreased the soil exchangeable Cd by 48.9%-55.0%, while the Fe/Mn oxides-bound Cd and residual Cd increased significantly by 19.6%-20.3% and 50.7%-69.4%, respectively. Pot experiment also suggested that both the APRs treatments (SL- and HA-APRs) significantly diminished soil Cd accumulation in rice (by 72.7% and 62.8%) coupling with the significantly decreased P leaching. These results provide a sustainable way to explore a novel cost-effective, high-efficient and bi-functional mineral-based soil amendments for environmental remediation.