Unveiling the barriers of Cd translocation from soil to rice: Insights from continuous flooding.

Understanding the spatiotemporal processes governing Cd behavior at the soil-solution-root interface is crucial for developing effective remediation strategies. This study examined the processes of chemical remediation in Cd-contaminated paddy soil using rhizotrons over the entire rice growth period. One-dimensional profile sampling with a 10 cm resolution revealed that during the initial flooding, paddy soil was strongly stimulated, followed by stabilization of porewater properties. X-ray diffraction of freeze-dried porewater confirmed the generation of submicron-precipitates such as CdS under continuous flooding, resulting in low ion levels of water-soluble Cd (<1 µg/L) and sulfate (<10 mg/L) in porewater. Two-dimensional imaging technologies indicated the maximum iron­manganese plaque (IP) within 20-110 µm of the root surface. Subsequently, monitoring O2 in the rhizosphere with a planar optode by two 100 cm2 membranes for a consecutive month revealed significant circadian O2 variations between the root base and tip. Destructive sampling results showed that acid-soluble Cd in soils, as available Cd, is crucial for Cd uptake by rice roots under continuous flooding. The IP deposited on the root surface, as the barriers of Cd translocation, increased with rice growth and blocked Cd translocation from soil to rice by about 18.11 %-25.43 % at maturity. A Si-Ca-Mg compound amendment reduced available Cd by about 10 % and improved Cd blocking efficiency by about 7.32 % through increasing IP concentration, resulting in the absorption ratio of Cd in the amendment group being half that of the control group. By unveiling the complex Cd interactions at the soil-rice interface, this study lays the groundwork for developing effective agricultural practices to mitigate Cd-contaminated paddy and ensure food safety.

Distinct biophysical and chemical mechanisms governing sucrose mineralization and soil organic carbon priming in biochar amended soils: evidence from 10 years of field studies.

While many studies have examined the role of biochar in carbon (C) accrual in short-term scale, few have explored the decadal scale influences of biochar on non-biochar C, e.g., native soil organic C (SOC) and added substrate. To address this knowledge gap, soils were collected from decade-old biochar field trials located in the United Kingdom (Cambisol) and China (Fluvisol), with each site having had three application rates (25-30, 50-60 and 75-100 Mg ha-1) of biochar plus an unamended Control, applied once in 2009. We assessed physicochemical and microbial properties associated with sucrose (representing the rhizodeposits) mineralization and the priming effect (PE) on native SOC. Here, we showed both soils amended with biochar at the middle application rate (50 Mg ha-1 biochar in Cambisol and 60 Mg ha-1 biochar in Fluvisol) resulted in greater substrate mineralization. The enhanced accessibility and availability of sucrose to microorganisms, particularly fast-growing bacterial genera like Arenimonas, Spingomonas, and Paenibacillus (r-strategists belonging to the Proteobacteria and Firmicutes phyla, respectively), can be attributed to the improved physicochemical properties of the soil, including pH, porosity, and pore connectivity, as revealed by synchrotron-based micro-CT. Random forest analysis also confirmed the contribution of the microbial diversity and physical properties such as porosity on sucrose mineralization. Biochar at the middle application rate, however, resulted in the lowest PE (0.3 and 0.4 mg of CO2-C g soil-1 in Cambisol and Fluvisol, respectively) after 53 days of incubation. This result might be associated with the fact that the biochar promoted large aggregates formation, which enclosed native SOC in soil macro-aggregates (2-0.25 mm). Our study revealed a diverging pattern between substrate mineralization and SOC priming linked to the biochar application rate. This suggests distinct mechanisms, biophysical and physicochemical, driving the mineralization of non-biochar carbon in a field where biochar was applied a decade before. Supplementary Information: The online version contains supplementary material available at 10.1007/s42773-024-00327-0.

Desulfurization steel slag improves the saline-sodic soil quality by replacing sodium ions and affecting soil pore structure.

Flue gas desulfurization steel slag (DS), a solid waste produced by coal power plants and steelworks, was proposed as an amendment for the remediation of saline-sodic soil. A pot experiment including three dosages of DS alone (1%, 5%, 10% w/w) and their combination with fulvic acid (FA, 1%, w/w) was conducted to evaluate the potentials of DS as an amendment and to explore remediation mechanism of DS combined with FA on saline-sodic soil. The soil salinity, nutrition, pore structure, water retention, consistency, and desiccation cracking of DS and FA-amended soils were determined. Application of DS resulted in a significant reduction of pH, sodium adsorption ratio (SAR), and exchangeable sodium percentage (ESP) of saline-sodic soil. The DS amendment significantly increased the 6-15 µm pore volume of soil. The combination application of DS and FA showed better effect than the DS alone. The DS amendments at 5% and 10% significantly increased the field water capacity, permanent wilting point, and available water content of the soil, whereas significantly decreased the plastic limit, liquid limit, and plastic index. The DS alone and combined with FA could effectively control the development of desiccation cracking, reduced significantly the crack area density and average width of cracks of the soil. Consequently, the improvement of alkalinity and soil physical properties by DS amendment significantly increased the yield of alfalfa grown on saline-sodic soil. The remarkable improvement of physical properties of saline-sodic soil contributed to the decrease of SAR and ESP by the Ca2+ in DS replacing the Na + at soil colloid sites. Our results suggested that DS amendments alone or combined with fulvic acid have great potential as saline-alkali soil amendment.

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.

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.

Heavy metal pollution risk of desulfurized steel slag as a soil amendment in cycling use of solid wastes.

The by-product of wet flue gas desulfurization, desulfurized steel slag (DS), had chemical characteristics like natural gypsum that can be used to improve saline-sodic soil. However, contamination risk of heavy metals for cycling utilization of DS in agriculture was concerned mostly. Both pot and field experiments were conducted for evaluating the potential pollution risk of DS as the amendment of saline-sodic soil. Results showed that application of DS decreased the contents of Cd, Cu, Zn, and Pb, while significantly increasing chromium (Cr) content in DS-amended soils. The field experiment demonstrated that the migration of heavy metals (Cd, Zn, Cu, and Pb) in the soil profile was negligible. The application of DS at the dosage of 22.5-225 tons/ha significantly increased the Cr content in alfalfa (Medicago sativa L.) but lower than the national standard for feed in China (GB 13078-2017). DS altered the chemical fraction of heavy metals (Zn, Cu, and Pb), transferred exchangeable, reducible into oxidizable and residual forms in DS-amended soil. Application of DS combined with fulvic acid (FA) could effectively reduce the movement of heavy metals in soil and the accumulation of Cr in alfalfa. Based on our results, DS was a safe and feasible material for agricultural use and presented relatively little pollution risk of heavy metals. However, the results also showed that DS to a certain extent had a potential environmental risk of Cr if larger dosages of DS were used.

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.

[Effects of Oyster Shell Powder and Lime on Availability and Forms of Phosphorus and Enzyme Activity in Acidic Paddy Soil].

Soil acidification improvement in the main grain production regions of southern China is an important issue to enhance the quality of cultivated land and promote grain yield. In order to explore the effects of oyster shell powder and lime on acidity and availability and inorganic forms of phosphorus in acidic paddy soil, a pot experiment was performed using oyster shell powder and lime amendments with dosages of 0.05%, 0.10%, and 0.15%. The results showed that both oyster shell powder and lime significantly (P<0.05) increased the pH and decreased exchangeable acid content of paddy soil. The improvement effects increased with the dosage of soil amendments. Under the same amendment dosage, the effects of lime on soil pH and acidity were higher than those of oyster shell powder. Both lime and oyster shell powder significantly increased the content of available P extracted using H2SO4-P, Bray-1 P, and Olsen-P techniques. The contents of inorganic P in soils decreased in the order of Fe-P>Al-P>Ca-P. The application of lime and oyster shell powder significantly increased the contents of Al-P and Fe-P in soil. Compared with the control treatment, lime and oyster shell power increased Al-P and Fe-P by 26.3%-37.4% and 7.9%-23.7%, respectively. However, there was no significant difference in Al-P content among treatments of the three amendment dosages. The contents of Fe-P and Ca-P in soils increased with an increased dosage of amendments. The activities of DHA, ALP, and IPP and the copy number of the phoD gene in soil increased with the application of lime and oyster shell powder, whereas the activities of ACP and the copy numbers of phoC and pqqC decreased. The application of lime and oyster shell powder at a rate of 0.10% and 0.15% significantly (P<0.05) increased the yield of rice. The lime and oyster shell powder treatments at the dosage of 0.15% increased rice yield by 34.2% and 46.8%, respectively, whereas the amendment had no significant effect on straw biomass of the rice crop. Correlation analysis showed that soil pH and the ALP activity were significantly positively correlated with inorganic P and available P contents, respectively. These results suggested that lime and oyster shell power could effectively increase the content of available phosphorus by eliminating soil acidity and improving the phosphatase activity, which played a positive role in increasing crop yield.

[Remediation Effect and Mechanism of Inorganic Passivators on Cadmium Contaminated Acidic Paddy Soil].

Cadmium (Cd) is one of the main pollutants in acidic paddy fields, and its accumulation in rice (Oryza sativa L.) and subsequent transfer to the food chain is an important environmental issue in China. In our field study, three types of inorganic passivators (silicon-calcium-magnesium-potassium fertilizer (SCMK), calcium magnesium phosphate fertilizer (CMP), and lime (L) at the rate of 750, 1500, and 2250 kg·hm-2, respectively) were applied to acidic paddy soils polluted by the heavy metal Cd in southern Zhejiang province. The objective of this study was to reveal the effects and chemical mechanisms of passivators on soil acidification and Cd accumulation in rice. The field experimental results showed that the three passivators could effectively improve soil acidification and reduce Cd accumulation in rice grains. The application of 2250 kg·hm-2 SCMK, CMP, and L increased soil pH by 0.62, 0.65, and 0.86 units; decreased exchangeable acidity by 67%, 69%, and 78%; and reduced the content of Cd in brown rice by 73%, 68%, and 77%, respectively. The application of 2250 kg·hm-2 SCMK, CMP, and L reduced the content of Cd in brown rice planted on polluted paddy rice fields to lower than 0.2 mg·kg-1, which reached the national food safety standard. Compared with the control, the application of SCMK, CMP, and L significantly (P<0.05) decreased the content of available Cd extracted by DTPA; decreased the contents of weak acid-extractable (F1) and reducible (F2) Cd; and increased the content of residual (F4) Cd. Correlation analyses indicated that Cd content in brown rice was significantly negatively correlated with soil pH and exchangeable cation content and significantly positively correlated with DTPA-Cd, weak acid-extractable (F1) and reducible (F2) Cd, and exchangeable Al contents. The partial least squares path model (PLS-PM) was used to analyze the relationship between the Cd content of brown rice, DTPA-Cd, and various chemical forms of Cd and soil properties. The direct path coefficients of soil exchangeable cations on Cd content in brown rice, available cadmium, and rice yield were -0.566, -0.866, and 0.873, respectively. Soil pH indirectly affected Cd content of brown rice mainly by directly affecting available Cd in soil. Field experiments demonstrated that the three passivators SCMK, CMP, and L were effective technologies for the safe production of rice in acidic paddy soils polluted by Cd. The possible mechanism for passivators reducing the bioavailability of Cd in soil and its accumulation in brown rice contributed to increased exchangeable cations in the soils. These findings could provide a scientific basis for the safe production of rice in acidic paddy soil polluted by heavy metals.

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.

Rice often accumulates higher Cd from contaminated soils, thereby endangering human health. In this study, microbial organic fertilizer (MOF) was applied at the rate of 3, 4.5, and 7.5 t·MOF·ha-1, respectively, to passivate Cd in polluted soils. The goals of the field experiments were to understand how MOF reduces the uptake of Cd in rice by affecting the mobility and bioavailability of Cd in the rhizosphere soil. BCR sequential extraction analysis recorded that the addition of MOF decreased the content of available Cd and increased Cd residual fraction in soils. Compared with the control treatment, the application of 7.5 t·MOF·ha-1 significantly increased the yield of rice by 7.9% and decreased the Cd content in brown rice by 86.4%. The application of MOF strengthened the oxidation of iron by increasing the relative abundance of Fe-oxidizing bacteria (FeOB) Thiobacillus, and further increased the ratio of amorphous/dissociative iron oxides (Feo/Fed) and thickened the iron plaques on the root surface of rice. The spatial distribution of Cd and Fe on rice root indicated the key role of iron plaques in preventing Cd from entering rice. The structural equation model confirmed that MOF application regulated iron oxides by FeOB, dehydrogenase activity, and catalase activity, thereby reducing the Cd uptake of rice.

Soil acidification induced decline disease of Myrica rubra: aluminum toxicity and bacterial community response analyses.

The decline disease of Myrica rubra tree is commonly induced by soil acidification, which affects the yield and the quality of fruits. It is hypothesized that aluminum toxicity and microbial community changes caused by soil acidification were the main causes of decline of Myrica rubra tree. In order to explore the decline mechanism of Myrica rubra tree, soils around healthy and decline trees of Myrica rubra were collected to compare the concentrations of different aluminum forms, enzyme activities, and bacterial community structure. In this study, soil samples were collected from the five main production areas of Myrica rubra, Eastern China. The results showed that diseased soils had higher exchangeable aluminum, lower enzyme activities, and lower microbial diversity than healthy soils at various sites. The toxic Al significantly decreased bacterial diversity and altered the bacterial community structure. The diseased soils had significantly lower α-diversity indices (ACE, Chao1, and Shannon) of bacterial community. The Al toxicity deceased the relative abundance of Acidobacteria and Planctomycetes, while enhanced the relative abundance of Cyanobacteria, Bacteroidetes, and Firmicutes in soils. Co-occurrence network analysis indicated that the Al toxicity simplified the bacterial network. The soil ExAl content was significantly and negatively correlated with the nodes (r = -0.69, p < 0.05) and edges (r = -0.77, p < 0.01) of the bacterial network. These results revealed that the Al toxicity altered soil bacterial community structure, resulting in the decline disease of Myrica rubra tree, while highlighted the role of Al forms in the plant growth. This finding is of considerable significance to the better management of acidification-induced soil degradation and the quality of fruits.

Straw and straw biochar differently affect phosphorus availability, enzyme activity and microbial functional genes in an Ultisol.

Crop straw is commonly returned back to agricultural fields to improve soil nutrient status. In order to compare the effects of straw returning modes (direct and carbonization returning) on the phosphorus (P) availability in acidic soils and explore possible chemical and microbial mechanisms, a pot experiment was conducted. The rice straw, canola stalk at the rate of 1% (w/w) and their corresponding biochar produced by the same amount of straw at 350 °C and 550 °C were used, and two-season crops (rice and soybean) were planted. Results indicated that the content of available P in biochar-treated soils was significantly higher than in the straw-treated soils owing to the biochar soluble P and increased pH. Straw returning increased the activities of urease, sucrase and catalase more than biochar. Biochar mode significantly increased the activity of alkaline phosphatase (ALP), while decreased the acid phosphatase (ACP) relative to the straw mode. Likewise, there were a significant rise in the copy number of phoD gene and a drop of phoC in the biochar mode. The P functional genes (phoD, gcd and pqqC) had the higher copy numbers in soils with biochar made at 350 °C. Similarly, biochar made at 350 °C improved the yields of rice and soybean more effectively. Therefore, straw returning modes affected the availability of P differently via chemical and microbial pathways and the ALP regulated by phoD played a crucial role in the conversion of P. Results demonstrated that biochar returning had a larger impact on the availability of P and developed the effectiveness of crop production than the straw returning directly.

Comparative study on the potential risk of contaminated-rice straw, its derived biochar and phosphorus modified biochar as an amendment and their implication for environment.

Direct application of contaminated-rice straw (CRS) to soil can cause the secondary pollution in agricultural land because of high content of Cd in rice straw. This study employed biochar or modified biochar technique to reduce the potential pollution risk of Cd in CRS. In the pot experiment, the CRS, straw biochar prepared at 300 °C (B300) and 500 °C (B500), and phosphorus modified biochar pyrolyzed at 300 °C (PB300) and 500 °C (PB500) were added at dosage of 5% into three typical paddy soils. The results showed that CRS and its derived biochar could enhance soil pH, EC, Eh, organic carbon, exchangeable base cations (K+, Na+, Ca2+ and Mg2+), and available phosphate. The application of CRS, biochar and phosphorus modified biochar significantly increased the contents of total Cd in soils relative to control soil. Compared to CRS, the biochar application (especially the PB500) decreased the contents of 0.01M CaCl2-extractable Cd. The application of CRS significantly increased the content of exchangeable Cd fraction (F1), whereas biochar increased residual Cd content (F4). The biochar and phosphorous modified biochar significantly decreased the contents of bioavailable Cd in soils compared to CRS application. The increased soil pH and dissolve organic matter were found to be the main factors in reducing the release of Cd in biochar. The possible mechanisms of biochar in reducing bioavailability of Cd were to significantly increase soil pH, enhance the complexation of Cd ions, and promote the transformation of Cd from easily available to stable (residual) forms. It could conclude that conversion of contaminated rice straw into biochar was an efficient way to minimize Cd availability in soil and reduce the pollution risk of Cd in rice straw.

Crop straw-derived biochar alleviated cadmium and copper phytotoxicity by reducing bioavailability and accumulation in a field experiment of rice-rape-corn rotation system.

Biochar has the potential to control the bioavailability and transformation of heavy metals in soil, thereby ensuring the safe crop production. A three seasons field experiment was conducted to investigate the effect of crop straw-derived biochar on the bioavailability and crop accumulation of Cd and Cu in contaminated soil. Wheat straw biochar (WSB), corn stalk biochar (CSB), and rice husk biochar (RHB) were applied at the rate of 0, 1.125, and 2.25 × 104 kg ha-1, respectively. The results showed that all types of biochar significantly increased soil pH, organic carbon and cation exchangeable capacity (CEC), compared to the control. The reduction in DTPA extractable Cd and Cu contents was much greater under high dosage biochar application, with a prominence at RHB treatment throughout the three cropping seasons, compared to the control. Moreover, the biological accumulation of Cd and Cu in the grains of rapeseed and corn significantly decreased after biochar application. Linear regression also confirmed the effective role of biochar in controlling the translocation and accumulation of Cd and Cu due to their inactive bioavailability. In addition, the sequential extraction indicated that exchangeable fraction (EXF) of Cu and Cd had decreased, while residual fraction (RSF) had increased under all biochar amendments. Contrarily, the oxidizable fraction (OXF) of Cd decreased while OXF of Cu increased under biochar treatments. Biochar application, especially RHB, could be an effective measure to enhance Cd and Cu adsorption and immobilization in polluted soils and thereby reducing its uptake and translocation to crops.

Flue gas desulfurization (FGD) steel slag ameliorates salinity, sodicity, and adverse physical properties of saline-sodic soil of middle Yellow River, China.

Saline-sodic soil is considered the most important low-yield soil in arid and semi-arid regions. Flue gas desulfurization (FGD) steel slag is a kind of by-product from wet FGD process, in which steel slag powder replaces lime as sorbent of SO2 emitted from coal-fired power plants. It could potentially be used to ameliorate saline-sodic soil. In this study, a large-scale field experiment of applying FGD steel slag as a new amendment of saline-sodic soils was conducted in the middle Yellow River, Inner Mongolia, China. The FGD steel slag was applied at a rate of 180 t/ha in 2015, 2016, and 2018, respectively. After FGD steel slag application for 1, 3, and 4 years, the soil samples were collected. The saline-sodic field without FGD steel slag amendment was used as the control treatment (CK). Compared with control, the application of FGD steel slag significantly (p < 0.05) decreased soil pH, electric conductivity (EC), salt content, sodium adsorption ratio (SAR), and exchangeable sodium percentage (ESP) of surface soil in saline-sodic soil. However, FGD steel slag increased the EC and salt content at the lower depth of soil profile because of the salt accumulation leached from surface soil. The FGD steel slag significantly increased the concentration of Ca2+ and reduced the concentrations of Na+, Cl-, CO32-, and HCO3- ions. FGD steel slag was beneficial for improving adverse physical properties of saline-sodic soil. The application of FGD steel slag significantly reduced the plastic index, tensile strength, and the formation of cracking in saline-sodic soil. The FGD steel slag reduced surface area density of crack (Dc) and average crack width (AW) by 49.1% and 58.7%, compared with the control. The reduction of soil cracking was contributed to the release of Ca2+ from FGD steel slag to exchange the Na+ on the soil cation exchange sites, which decrease the clay dispersion in soil. The findings of this study confirmed that FGD steel slag could effectively and rapidly remediate saline-sodic soils through decreasing soil sodicity and improving poor physical properties.

Biochar derived from cadmium-contaminated rice straw at various pyrolysis temperatures: Cadmium immobilization mechanisms and environmental implication.

The total concentration, chemical speciation and availability of Cd in biochar derived from Cd-contaminated rice straw were determined to evaluate the potential environmental risk of Cd in biochar and the possibility of biochar as effective way to dispose Cd-contaminated straw. The Cd was concentrated with the increased pyrolysis temperature, while the bioavailability of Cd in biochar was reduced. The sequence extraction indicated that residual fraction of Cd increased and acid exchangeable fraction decreased as pyrolysis temperature increased. The biochar modified by phosphate could significantly reduce the concentration of total and DTPA-extractable Cd in biochar. The pot experiment demonstrated that pyrolysis reduced the potential environmental risk of Cd in biochar. The precipitation and co-precipitation, physical sorption, surface electrostatic interaction, and functional group complexation could be the potential mechanisms of Cd immobilization in biochar. These findings suggested that pyrolysis would be an acceptable and feasible way to dispose contaminated rice straw.

[Dynamic Effects of Direct Returning of Straw and Corresponding Biochar on Acidity, Nutrients, and Exchangeable Properties of Red Soil].

To compare the dynamic effects of straw and corresponding biochar on soil acidity, nutrients, and exchangeable capacity in red soil, a pot experiment was performed. The treatments included control (CK), rice straw (R1B0), rice straw biochar prepared at 350℃ (R1B1) and 550℃ (R1B2), rape stalk (R2B0), and rape stalk biochar prepared at 350℃ (R2B1) and 550℃ (R2B2). Straw at 1% and corresponding biochar were added to a strongly acidic red soil. The rice was planted as the experimental crop. Soils were collected at the seedling, tillering, filling and mature stages of rice growth, respectively. The changes in soil pH, exchangeable acidity, organic matter, nutrients (NH4+-N and NO3--N), and exchangeable cations in soils were measured. The results showed that soil pH, NH4+-N, and NO3--N concentrations decreased with the growth period of rice, while the organic matter content and cation exchange capacity (CEC) increased. Direct returning of straw and biochar could increase soil pH, organic matter content, and exchangeable cations content, and reduce the total amount of exchangeable acids. In the mature stage of rice, rice straw and rape stalk biochar at 350℃ increased the soil pH by 0.29 and 0.42, respectively, compared to the control treatment. Similarly, biochar decreased the exchangeable acidity and exchangeable Al3+ content significantly compared to the direct returning treatments of straw. The exchangeable acidity and exchangeable Al3+ contents of soils in R1B2 and R2B1 treatments decreased by 54.8% and 58.9%, respectively, compared to the control treatment. The soil organic matter (SOM) content and CEC in biochar treatments were significantly higher than those in direct returning treatments of straw. Overall, the effects of rape stalk biochar on soil properties were slightly stronger than those of rice straw. The correlation analysis showed that soil exchangeable acids had a significantly negative correlation with organic matter (R=-0.912, P<0.01), and CEC (R=-0.866, P<0.05). The CEC in soils was positively related to organic matter (R=0.833, P<0.05). Direct returning of straw and biochar applications can effectively improve soil acidity and increase nutrient contents. The effects of straw biochar on soils were stronger than the direct returning of straw in decreasing soil acidity, and increasing soil organic matter content and exchangeable capacity in acidic soils.

[Dynamic Effects of Different Biochars on Soil Properties and Crop Yield of Acid Farmland].

To investigate the dynamic effects of biochars produced from different biomass materials on farmland soil acidity, exchangeable cations, phosphorus nutrient, and crop yield, a field experiment was performed on acid paddy soil. Five types of biochars-rice straw biochar (RSB), maize straw biochar (MSB), wheat straw biochar (WSB), rice husk biochar (RHB), and bamboo charcoal (BCB)-were applied to farmland soil at mass fraction of 0.1%. No biochar addition was used as control treatment (CK). The soil physicochemical properties and crop yields were analyzed after harvesting rice, rapeseed, and corn crops. Results indicated that the addition of biochars could effectively increase soil pH and exchangeable cations and reduce exchangeable acid content, but the effects decreased with time. The biochars increased the content of exchangeable K+, Ca2+, and Mg2+ and decreased the exchangeable Na+ content in soils. The biochars increased the contents of organic matter (SOM), available phosphorus, total phosphorus, and inorganic phosphorus (Al-P and Fe-P). Compared with the control treatment, biochars significantly (P<0.05) increased the yields of rice, oil seed, and maize crops. Rice husk biochar (RHB) had the best effect in improving acid soil physicochemical properties and increasing crop yield.

Predicting accumulation of Cd in rice (Oryza sativa L.) and soil threshold concentration of Cd for rice safe production.

Rice contamination by cadmium (Cd) poses a serious threat to human health, which has attracted widespread concerns in China. It is imperative to determine major soil factors influencing the accumulation of Cd in rice and develop prediction models to derive the threshold concentration of Cd in soil for rice food safety. In this study, the bioavailability, accumulation, and transfer of Cd in the 18 typical paddy soil-rice systems with a wide range of soil properties was investigated using pot experiments. The regression-based models incorporated with total or extractable Cd and soil properties were constructed to predict Cd content of rice grain. Pot experimental results indicated that rice showed a high accumulation potential for Cd, while rice grains grown in acid soils displayed larger Cd contents than those in neutral and alkaline soils. The pH and MnO content were major soil factors influencing the Cd accumulation of rice. Multiple regression models based on the total Cd, extractable Cd, pH, and MnO content in soils could well describe the Cd content in rice grain. Measured Cd content of rice grains from field samples demonstrated that the empirical models could quantitatively predict the Cd content of rice grains. The threshold concentrations of Cd in soils for rice food safety could be back-calculated by both EDTA-extractable and total Cd contents in soils. The EDTA-extractable Cd in soils could use as an indication to derive the threshold concentrations of Cd for rice food safety. In conclusions, multiple regression models proved reliable and practical in predicting Cd accumulation in rice grain. These empirical models could well predict the content of Cd in rice grain and deduce soil Cd threshold criteria. These results could help to quantitatively evaluate the health risk of Cd accumulation in rice crop and provide a useful reference for safe production of rice.

Si-Ca-K-Mg amendment reduces the phytoavailability and transfer of Cd from acidic soil to rice grain.

Cadmium (Cd) contamination in the soil-rice chain is the major threat to human health in China. It is very necessary to lower Cd phytoavailability in contaminated soils and reduce Cd transfer from soil to rice for food safety. This study applied the Si-Ca-K-Mg amendment (SCKM) to immobilize Cd in acidic soils and then reduce its accumulation in rice grain (Oryza sativa L.). Two agricultural soils (Alfisol and Ultisol) collected from Eastern China were treated with three levels of Cd concentration (0, 0.4, and 2.0 mg/kg), respectively, for pot experiment. The phytoavailability and chemical forms of Cd in two soils were determined using ethylenediaminetetraacetic acid (EDTA) and the European Community Bureau of Reference (BCR) extraction procedures. At 2.0 mg Cd/kg-treated soils, application of SCKM amendment increased the yield of rice grain by 10-17% for Alfisol and 14-39% for Ultisol, and reduced the concentrations of EDTA-extractable Cd by 6-27% for Alfisol and 5-25% for Ultisol, compared with treatment without amendment. SCKM amendment significantly (p < 0.05) reduced the bioconcentration factor (BCF) of Cd in root, straw, and grain of rice. Compared with treatment without amendment, the application of amendments decreased the Cd concentrations of rice grains by 35-76% for Alfisol and 31-72% for Ultisol, respectively. The BCR sequential extraction revealed that amendment reduced acid soluble Cd fraction by 6.2-13.6% for Alfisol and 6.1-13.5% for Ultisol, respectively, indicating that amendment could effectively transform the highly phytoavailable Cd into a more stable form. SCKM amendment addition significantly (p < 0.05) increased soil pH and exchangeable K+, and decreased exchangeable Al3+ contents in both soils. Our results demonstrated that SCKM amendment was effective in reducing the phytoavailability and transfer of Cd in soil-rice system, and ameliorating soil acidity. The SCKM amendment had greater potential as a low-cost and friendly environmentally amendment for safe production of rice in Cd-contaminated soils.