Effects of microbial organic fertilizer (MOF) application on cadmium uptake of rice in acidic paddy soil: Regulation of the iron oxides driven by the soil microorganisms

Intensive ammonium fertilizer addition activates iron and carbon conversion coupled cadmium redistribution in a paddy soil under gradient redox conditions

While over-fertilization and nitrogen deposition can lead to the enrichment of nitrogen in soil, its effects on heavy metal fractions under gradient moisture conditions remains unclear. Here, the effect of intensive ammonium (NH4+) addition on the conversion and interaction of cadmium (Cd), iron (Fe) and carbon (C) was studied. At relatively low (30-80 %) water hold capacity (WHC) NH4+ application increased the carbonate bound Cd fraction (F2Cd), while at relatively high (80-100 %) WHC NH4+ application increased the organic matter bound Cd fraction (F4Cd). Iron‑manganese oxide bound Cd fractions (F3Cd) and oxalate-Fe decreased, but DCB-Fe increased in NH4+ treatments, indicating that amorphous Fe was the main carrier of F3Cd. The variations in F1Cd and F4Cd observed under the 100-30-100 % WHC treatment were similar to those observed under low moisture conditions (30-60 % WHC). The C=O/C-H ratio of organic matter in soil decreased under the 30-60 % WHC treatment, but increased under the 80-100 % WHC treatment, which was the dominant factor influencing F4Cd changes. The conversion of NH4+ declined with increasing soil moisture content, and the impact on oxalate-Fe was greater at 30-60 % WHC than at 80-100 % WHC. Correspondingly, genetic analysis showed the effect of NH4+ on Fe and C metabolism at 30-60 % WHC was greater than at 80-100 % WHC. Specifically, NH4+ treatment enhanced the expression of genes encoding extracellular Fe complexation (siderophore) at 30-80 % WHC, while inhibiting genes encoding Fe transmembrane transport at 30-60 % WHC, indicating that siderophores simultaneously facilitated Cd detoxification and Fe complexation. Furthermore, biosynthesis of sesquiterpenoid, steroid, butirosin and neomycin was significantly correlated with F4Cd, while glycosaminoglycan degradation metabolism and assimilatory nitrate reduction was significantly correlated with F2Cd. Overall, this study gives a more comprehensive insight into the effect of NH4+ on activated Fe and C conversion on soil Cd redistribution under gradient moisture conditions.

Cadmium biosorption and mechanism investigation using two cadmium-tolerant microorganisms isolated from rhizosphere soil of rice

Microbial remediation of cadmium-contaminated soil offers advantages like environmental friendliness, cost-effectiveness, and simple operation. However, the efficacy of this remediation process relies on obtaining dominant strains and a comprehensive understanding of their Cd adsorption mechanisms. This study identified two Cd-resistant bacteria, Burkholderia sp. 1-22 and Bacillus sp. 6-6, with significant growth-promoting effects from rice rhizosphere soil. The strains showed remarkable Cd resistance up to ∼200 mg/L and alleviated Cd toxicity by regulating pH and facilitating bacterial adsorption of Cd. FTIR analysis showed crucial surface functional groups, like carboxyl and amino groups, on bacteria played significant roles in Cd adsorption. The strains could induce CdCO3 formation via a microbially induced calcium precipitation (MICP) mechanism, confirmed by SEM-EDS, X-ray analysis, and elemental mapping. Pot experiments showed these strains significantly increased organic matter and enzyme activity (e.g., urease, sucrase, peroxidase) in the rhizosphere soil versus the control group. These changes are crucial for restricting Cd mobility. Furthermore, strains 6-6 and 1-22 significantly enhance plant root detoxification of Cd, alleviating toxicity. Notably, increased pH likely plays a vital role in enhancing Cd precipitation and adsorption by strains, converting free Cd into non-bioavailable forms.

Adsorption mechanism of Cd2+ on microbial inoculant and its potential for remediation Cd-polluted farmland soils

The adsorption characteristics and mechanism of Cd2+ on microbial inoculant (MI) mainly composed of Bacillus subtilis, Bacillus thuringiensis and Bacillus amyloliquefaciens, and its potential for remediation Cd polluted soils through batch adsorption and soil incubation experiments. It was found that the Freundlich isotherm model and the pseudo-second-order kinetics were more in line with the adsorption processes of Cd2+. The maximum adsorption capacity predicted by Langmuir isotherm model suggested that of MI was 57.38 mg g-1. Scanning electron microscopy and energy dispersive spectroscopy (SEM-EDS) images exhibited the surface structure of MI was damaged to varying degrees after adsorption, and Cd element was distributed on the surface of MI through ion exchange. X-ray diffraction (XRD) results showed that CdCO3 was formed on the surface of MI. Moreover, the functional groups (-OH, C-H, and -NH) involved in the adsorption of Cd2+ through fourier transform infrared spectroscopy (FTIR). After applying MI to Cd-contaminated soil, it was found that soil pH, conductivity (EC) and soil organic matter (SOM) increased by 0.84 %-2.43 %, 31.6 %-241.48 %, and 8.11 %-24.1 %, respectively, when compared with the control treatments. The content of DTPA-Cd in the soils was significantly (P < 0.05) reduced by 15.48 %-29.68 % in contrast with CK, and the Cd speciation was transformed into a more stable residual fraction. The activities of urease, phosphatase and sucrose were increased by 3.5 %-45.18 %, 57.00 %-134.18 % and 52.51 %-70.52 %, respectively, compared with CK. Therefore, MI could be used as an ecofriendly and sustainable material for bioremediation of Cd-contaminated soils.

Predicting Cd accumulation in rice and identifying nonlinear effects of soil nutrient elements based on machine learning methods

The spatial mismatch of Cd content in soil and rice causes difficulties in environmental management for paddy soil. To investigate the influence of soil environment on the accumulation of Cd in rice grain, we conducted a paired field sampling in the middle of the Xiangjiang River basin, examining the relationships between soil properties, soil nutrient elements, Cd content, plant uptake factor (PUFCd), and translocation factors in different rice organs (root, shoot, and grain). The total soil Cd (CdT) and available Cd (CdA) contents and PUFCd showed large spatial variability with ranges of 0.31-6.19 mg/kg, 0.03-3.07 mg/kg, and 0.02-3.51, respectively. Soil pH, CdT, CdA, and the contents of soil nutrient elements (Mg, Mn, Ca, P, Si, and B) were linearly correlated with grain Cd content (Cdg) and PUFCd. The decision tree analysis identified nonlinear effects of Si, Zn and Fe on rice Cd accumulation, which suggested that low Si and high Zn led to high Cdg, and low Si and Fe caused high PUFCd. Using the soil nutrient elements as predictor variables, random forest models successfully predicted the Cdg and PUFCd and performed better than multiple linear regressions. It suggested the impacts of soil nutrient elements on rice Cd accumulation should receive more attention.

Effects of elevation and geomorphology on cadmium, lead and chromium enrichment in paddy soil and rice: A case study in the Xiangtan basin of China

The distributions of heavy metals in paddy fields and rice along river valleys were studied to explore the key factors affecting the accumulation of heavy metals in the upstream terraces and downstream plains. Results from 975 sampling sites showed that elevation, growing season and soil organic matter (OM) had significant effects on the content of Cd and Pb in topsoil and rice. The content of Cd (0.47-0.66 mg kg-1) and Pb (49.9-68.6 mg kg-1) in paddy fields with low elevation (30-60 m) in the downstream plains was significantly higher than the content of Cd (0.29-0.38 mg kg-1) and Pb (43.9-56.3 mg kg-1) in the upstream terraces with high altitude (60-90 m). In the double-rice production area, late rice generally produced grains with higher Cd and Pb content than early rice. Soil Cd was positively increased with the content of OM, especially in the downstream plains. When elevation was used for principal component analysis, plains with low elevation were grouped together with high content of total and soluble Cd, OM and Pb in soil, as well as high content of Cd and Pb in late rice. Altitude is one of the key factors affecting Cd content in rice. Although content of Cr (93.7-138.0 mg kg-1) was significantly higher than that of Cd and Pb in soil, content of Cr was lower than that of Cd in rice. These results indicate that paddy fields with elevation of 30-60 m in the downstream plains had high risk to produce late rice with Cd and Pb content exceeding the food safety standard 0.2 mg kg-1, which may be resulted from the driving force of runoff on soil soluble Cd and Pb from terraces to alluvial plains in river valleys.

Restricted colloidal-bound phosphorus release controlled by alternating flooding and drying cycles in an alkaline calcareous soil

Colloid-facilitated phosphorus (P) migration plays an important role in P loss from farmland to adjacent water bodies. However, the dynamics of colloidal P (Pcoll) release as influenced by irrigation in alkaline calcareous soil remains a knowledge gap. The present study, monitored the dynamic change of Pcoll under different water management strategies: 1) control, 2) flooding, and 3) alternating flooding and drying cycles. Soil water-dispersible colloids (0.6 nm-1 μm) were extracted by combining filtration and ultrafiltration methods. The contents of P, cation and organic carbon in the water-dispersible colloids were determined and the stability and mineral composition of colloidal fractions were characterized. The results showed that Pcoll ranged from 16.5 to 25.5 mg kg-1 and represented 42.8%-64.9% of the water-extracted P in the control. Flooding significantly decreased the Pcoll content by 16.0%-62.1% (mean 32.7%) and it may be attributed to the dissolution of colloidal iron (Fe) bound P. The alternating flooding and drying treatment significantly reduced the Pcoll content by 11.6%-88.0% (mean 67.6%). The Pcoll content of the flooding event was always greater than the Pcoll content of the drying event during flooding and drying cycles. Redundancy analysis and random forest modeling showed that the colloidal calcium (Ca) and ionic strength in soil solutions had negative correlations with the Pcoll content, and pH, ionic strength and truly dissolved P were the critical factors affecting Pcoll. Drying of the flooded soil led to the decrease of pH and the increase of ionic strength, colloidal Ca content and positive charges of colloid surfaces, which promoted colloid aggregation and enhanced soil P sorption capacity. This restricted the loss potential of Pcoll. In summary, controlled flooding and drainage when managed correctly have a role to play in mitigating Pcoll loss from P-enriched calcareous soils.

Soil inorganic amendments produce safe rice by reducing the transfer of Cd and increasing key amino acids in brown rice

The digestibility of cadmium (Cd) in brown rice is directly related to amino acid metabolism in rice and human health. In our field study, three kinds of alkaline calcium-rich soil inorganic amendments (SIAs) at three dosages were applied to produce safe rice and improve the quality of rice in Cd-contaminated paddy. With the increased application of SIA, Cd content in iron plaque on rice root significantly increased, the transfer of Cd from rice root to grain significantly decreased, and then Cd content in brown rice decreased synchronously. The vitro digestibility of Cd in brown rice was estimated by a physiologically based extraction test. Results showed that more than 70% of Cd in brown rice could be digested by simulated gastrointestinal juice. Based on the total and digestible Cd contents in brown rice to evaluate the health risk, the application of 2.25 ton SIA/ha could produce safe rice in acidic slightly Cd-contaminated paddy soils. The amino acids (AAs) in brown rice were determined by high-performance liquid chromatography. The contents of 5 key AAs (KAAs) that actively respond to environmental changes increased significantly with the increased application of SIA. The structural equation model indicated that KAAs could be affected by the Cd translocation capacity from rice root to grain, and consequently altered the ratio of indigestible Cd in brown rice. The formation of indigestible KAAs-Cd complexes by combining KAAs (phenylalanine, leucine, histidine, glutamine, and asparagine) with Cd in brown rice could be considered a potential mechanism for reducing the digestibility of Cd.

Role of iron manganese plaque in the safe production of rice (Oryza sativa L.) grains: Field evidence at plot and regional scales in cadmium-contaminated paddy soils

The relationship between iron manganese plaque (IP) and cadmium (Cd) accumulation by rice in the microenvironment of rice rhizosphere at varying field scales needs to be further explored. In this study, we selected different rice varieties and implemented tailored amendments to ensure the safe production of rice grains in heavily Cd-contaminated farmland situated around an E-waste dismantling site. Through regional surveys, we elucidated the role of IP in facilitating safe rice production. The selection of low-Cd accumulating rice varieties and application of appropriate amendments with sufficient dosages allowed for the effective reduction of Cd transport from soil to rice, resulting in a safe concentration of Cd in rice grains. Analysis using a random forest algorithm indicated that iron (Fe) played a more pivotal role than manganese in soil-rice systems in mitigating Cd accumulation in brown rice. The presence of Fe in IP (IP-Fe) at a low loading mass was unfavorable to the Cd-safe production of rice, while at an IP-Fe loading mass of 52 g/kg, the Cd content in brown rice decreased to a safe level. Furthermore, precipitation, coprecipitation, and complexation of surface functional groups contributed to Cd fixation on IP, as indicated by scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy, electron probe microanalysis, and Fourier-transform infrared spectroscopy with attenuated total reflection. Our results highlighted the key role of IP in the production of Cd-safe rice at different field scales.

Extreme low-temperature events can alleviate micronutrient deficiencies while increasing potential health risks from heavy metals in rice

Despite global warming, extreme low-temperature stress (LTS) events pose a significant threat to rice production (especially in East Asia) that can significantly impact micronutrient and heavy metal elements in rice. With two billion people worldwide facing micronutrient deficiencies (MNDs) and widespread heavy metal pollution in rice, understanding these impacts is crucial. We conducted detailed extreme LTS experiments with two rice (Oryza sativa L.) cultivars (Huaidao 5 and Nanjing 46) grown under four temperature levels (from 21/27 °C to 6/12 °C) and three LTS durations (three, six, and nine days). We observed significant interaction effects for LTS at different growth stages, durations and temperature levels on the contents and accumulation of mineral elements. The contents of most mineral elements (such Fe, Zn, As, Cu, and Cd) increased significantly under severe LTS at flowering, but decreased under LTS at the grain-filling stage. The accumulations of all mineral elements decreased at the three growth stages under LTS due to decreased grain weight. The contents and accumulation of mineral elements were more sensitive to LTS at the peak flowering stage than at the other two stages. Furthermore, the contents of most mineral elements in Nanjing 46 show larger variation under LTS compared to Huaidao 5. Accumulated cold degree days (ACDD, °C·d) were found to be suitable for quantifying the effects of LTS on the relative contents and accumulations of mineral elements. LTS at the flowering stage will help alleviate MNDs, but may also increase potential health risks from heavy metals. These results provide valuable insights for evaluating future climate change impacts on rice grain quality and potential health risks from heavy metals.

Key factors influencing arsenic phytotoxicity thresholds in south China acidic soils

Arsenic (As) toxicity threshold values (TTVs) for plants are fundamental to both establishing regional As reference values in soil and performing risk assessment. However, TTVs vary with plant species and soil types. In this study, a hydroponic experiment with 16 plant species was conducted to screen the most As-sensitive plant species. The results showed that the EC20 (available As concentration at which shoot biomass or height is inhibited by 20%) values were 1.38-104.4 mg L-1 for shoot height and 0.24-42.87 mg L-1 for shoot fresh biomass. Rice was more sensitive to As toxicity than the other species. Therefore, it was chosen as the ecological receptor in the pot experiment on As phytotoxicity in nine types of soils collected from Fujian Province in South China. The EC10 and EC20 with respect to rice shoot height were 3.72-29.11 mg kg-1 and 7.12-45.60 mg kg-1, respectively. Stepwise regression analysis indicated that free iron oxide concentration is the major factor that affects As bioavailability in soil, and ECx (x = 10, 20, and 50) of soil available As for shoot height was positively related to free iron oxide concentration in soil. In addition, soil cation exchange capacity, clay (<0.002 mm) content, and exchangeable magnesium content are also important factors influencing As phytotoxicity in acidic soils. The regression models can be used to predict As phytotoxicity in acidic soils.

Organic fertilization integrated with water management to remediate As and Cd contamination in a paddy soil

Soil heavy metal pollution is the main risk for sustainable agriculture, especially the combination of As and Cd pollution in paddy fields which may lead to the superimposed accumulation in rice. There is an urgent need for environmental-friendly and cost-effective strategies to remediate the contamination of As and Cd in soils. In this work, a pot culture experiment was conducted in a As and Cd polluted paddy soil to explore the effects of organic fertilization (OF) and two water managements (continuous flooding, CF; intermittent irrigation, II) on the fractionation of As and Cd in soil, and the uptake of As and Cd by rice. The results showed that OF integrated with intermittent irrigation performed best in reducing the contents of As and Cd in rice grains by 58.9 % and 69.3 %, respectively, under compound pollution. The significant conversion of available As and Cd to stable species (specifically adsorbed and Fe-Mn/Al oxide bound) under OF + II were supported by the changes in an array of soil attributes such as pH, Eh, soluble Fe and dissolved organic carbon (DOC). Intermittent irrigation was more conducive to the accumulation of As outside the roots, and Fe-plaque prevented As uptake by roots and the translocation to shoots. While more accumulation of Fe-plaque along with Cd on root surface induced by continuous flooding is helpful for depressed assimilation of Cd by rice. Considering the combined contamination of As and Cd polluted in paddy soils, a management approach was proposed based on intermittent irrigation and application of organic fertilizer at the rate of 0.1 % (∼ 2.3 t/ha) in two phases (two weeks before planting or drainage). Organic fertilization will hold great promise in restoring polluted soils and maintaining soil health via suppressing the lability of heavy metals and providing nutrients.

Inorganic amendments improve acidic paddy soils: Effects on soil properties, Al fractions, and microbial communities

Alkaline soil inorganic amendments (SIAs) have been extensively used to improve acidic soils. In this study, we arranged 9 treatments of low, medium, and high application dosages of silicon calcium magnesium potassium fertilizer, calcium magnesium phosphate fertilizer, and lime in the field to study the mechanism of SIAs in improving acidic soils. The Al sequential extraction experiment showed that the application of SIAs tended to transform from active to stable fractions of Al. By amplicon sequencing, it was observed that the application of SIAs significantly affected microbial community compositions in rhizosphere soils. With the decrease in soil acidity, the microbial function was also enhanced, especially the activity of dehydrogenase. In this study, the acidity-related indicators in soils (pH, exchangeable acid, and exchangeable base cations) were first integrated into an index-AIV (acidity improvement value), which was used to assess the relationship with other soil properties. The redundancy analysis and correlation network between soil chemical and biological indexes indicated that SIAs did not greatly affect the fungi community structure, while greatly increased or decreased the abundance of bacteria, especially Acidobacteria, Nitrospirae, and Crenarchaeota. Our data revealed the SIAs optimized soil environment for rice growth jointly by decreasing Al mobility, improving soil microbial function, and increasing soil fertility.

Different composites inhibit Cd accumulation in grains under the rice-oilseed rape rotation mode of karst area: A field study

Ensuring the safe production of food and oil crops in soils with elevated cadmium (Cd) content in karst regions is crucial. We tested a field experiment to examine the long-term remediation effects of compound microorganisms (CM), strong anion exchange adsorbent (SAX), processed oyster shell (POS), and composite humic acids (CHA) on Cd contamination in paddy fields under a rice-oilseed rape rotation system. In comparison to the control group (CK), the application of amendments significantly increased soil pH, cation exchange capacity (CEC), and soil organic matter (SOM) content while markedly decreasing the content of available Cd (ACd). During the rice cultivation season, Cd was predominantly concentrated in the roots. Relative to the control (CK), the Cd content in each organ was significantly reduced. The Cd content in brown rice decreased by 19.18-85.45%. The Cd content in brown rice following different treatments exhibited the order of CM > POS > CHA > SAX, which was lower than the Chinese Food Safety Standard (GB 2762-2017) (0.20 mg/kg). Intriguingly, during the oilseed rape cultivation season, we discovered that oilseed rape possesses potential phytoremediation capabilities, with Cd mainly accumulating in roots and stems. Notably, CHA treatment alone significantly decreased the Cd content in oilseed rape grains to 0.156 mg/kg. CHA treatment also maintained soil pH and SOM content, consistently reduced soil ACd content, and stabilized Cd content in RSF within the rice-oilseed rape rotation system. Importantly, CHA treatment not only enhances crop production but also has a low total cost (1255.230 US$/hm2). Our research demonstrated that CHA provides a consistent and stable remediation effect on Cd-contaminated rice fields within the crop rotation system, as evidenced by the analysis of Cd reduction efficiency, crop yield, soil environmental change, and total cost. These findings offer valuable guidance for sustainable soil utilization and safe production of grain and oil crops in the context of high Cd concentrations in karst mountainous regions.

Immobilization of Cd by mercapto-palygorskite in typical calcareous and acidic soil aggregates: Performance and differences

The microscale spatial heterogeneity and complexity of soil aggregates affect the properties and distribution of heavy metals (HMs). It has been confirmed that amendments can alter the distribution of Cd in soil aggregates. However, whether the Cd immobilization effect of amendments varies across soil aggregate levels remains unknown. In this study, soil classification and culture experiments were combined to explore the effects of mercapto-palygorskite (MEP) on Cd immobilization in soil aggregates of different particle sizes. The results showed that a 0.05-0.2% MEP application decreased soil available Cd by 53.8-71.62% and 23.49-36.71% in calcareous and acidic soils, respectively. The Cd immobilization efficiency of MEP in calcareous soil aggregates was in the following order: micro-aggregates (66.42-80.19%) > bulk soil (53.78-71.62%) > macro-aggregates (44.00-67.51%), while the efficiency in acidic soil aggregates was inconsistent. In MEP-treated calcareous soil, the percentage change in Cd speciation in micro-aggregates were higher than that in macro-aggregates, whereas there was no significant difference in Cd speciation between the four acidic soil aggregates. Mercapto-palygorskite addition in micro-aggregates of calcareous soil increased the available Fe and Mn concentrations by 20.98-47.10% and 17.98-32.66%, respectively. Mercapto-palygorskite had no effect on soil pH, EC, CEC, and DOC values, while the difference in soil properties between the four particle sizes was the main influencing factor of MEP treatments on Cd in calcareous soil. The effects of MEP on HMs varied across soil aggregates and soil types, but had strong specificity and selectivity for Cd immobilization. This study illustrates the influence of soil aggregates on Cd immobilization using MEP, which can be used to guide the remediation of Cd-contaminated calcareous and acidic soils.

Prediction model for Cd accumulation of rice (Oryza sativa L.) based on extractable Cd in soils and prediction for high Cd-risk regions of southern Zhejiang Province, China

Soil environmental quality in China for agricultural land always considers the effect of total cadmium (Cd) in soil, ignoring the bioavailability of soil Cd. The 139 paired rice (Oryza sativa L.) and soil samples were collected from the Cd-contaminated paddy fields of southern Zhejiang Province, China. The objectives of this study were to establish accurate prediction models for Cd accumulation in brown rice based on bioavailable Cd and physiochemical properties of soils and to evaluate the safety of rice production in Cd-contaminated paddy. The bioavailable Cd in soils was extracted and evaluated by using CaCl₂, HNO₃, diethylenetriamine pentaacetic acid (DTPA), diffusive gradients in thin-films technique (DGT), and sequential extraction method proposed by the European Community Bureau of Reference; 100 pairs of data were used as training sets, and the remaining 39 sets were used as validation sets. Stepwise multiple linear regression analysis and random forest analysis showed that total Cd in soil could roughly indicate the content of Cd in rice, while extractable Cd could better explain the accumulation of Cd in rice grain and DTPA and DGT extractive technologies are the most evaluative. The validation sets also showed that the prediction model has a good fit. Based on the prediction model for Cd in brown rice based on soil pH and DGT extractive Cd, the Monte Carlo simulation showed that 74.32% and 89.35% of the estimated brown rice hazard quotient (HQ) of the daily Cd intake of adults and children in safe utilization paddy sites could exceed the safe level of 1, respectively. Additionally, the threshold values for extractable Cd by DGT or DTPA for rice safe production were 3.4 μg/kg or 0.13 mg/kg when the pH in soils was below 5.5. The results further proved the threshold concentration of extractable Cd for predicting high-risk soils of Cd contamination in brown rice.