Improving liming mode for remediation of Cd-contaminated acidic paddy soils: Identifying the optimal soil pH, model and efficacies.

Liming has been widely taken to remediate Cd-contaminated acidic paddy soils, whereas liming mode involving in the relevant optimal soil pH, model and efficacies remain unclear. Both soil and field liming experiments were conducted to improve liming mode for precise remediation of Cd-contaminated acidic paddy soils. Soil batch liming experiments indicated soil DTPA-Cd and CaCl2-Cd were piecewise linearly correlated to soil pH with nodes of 6.8-8.0, and decreased respectively by 15.3%37.7% and 80.7%93.8% (P < 0.05) when soil pH raised over the nodes, indicating an appropriate target soil pH 7.0 for liming. Stepwise linear regression revealed that liming ratio (LR, kg ha-1) could be estimated from soil basal pH (pH0) and the interval to the target soil pH (ΔpH), as [LR=exp(1.10 ×ΔpH+0.61 ×pH0-4.98), R2 = 0.97, n = 42, P < 0.01]. The model exhibited high prediction accuracy (95.2%), low mean estimation error (-0.02) and root mean square error (0.20). Field liming experiment indicated liming to target pH decreased respectively soil CaCl2-Cd by 95.2-98.0% and rice grain Cd by 59.8-80.6% (P < 0.01), whereas uninfluenced rice grain yield. Correlation analysis and structural equation models (SEM) demonstrated that great reduction in Cd phytoavailability was mainly attributed to the transformation of soil water-soluble and exchangeable Cd to carbonate-bound Cd and Fe/Mn oxides-bound Cd and reduced Cd in iron plaque as increasing soil pH. However, rice grain Cd of 50% samples met national food safety standards limit of China (0.2 mg kg-1) due to the high soil Cd level (0.8 mg kg-1). In conclusion, liming to target soil pH 7.0 could be considered as a precise and effective remediation mode for Cd-contaminated acidic paddy soils and complementary practices should be implemented for severe pollution. Our results could provide novel insights on precise liming remediation of Cd-contaminated acidic paddy soils.

Interspecific root interaction enhances cadmium accumulation in Oryza sativa when intercropping with cadmium accumulator Artemisia argyi.

The contamination of arable land with heavy metals, such as Cd, is a serious concern worldwide. Intercropping with Cd accumulators can be used for efficient safe crop production and phytoremediation of Cd-contaminated soil. However, the effect of intercropping on Cd uptake by main crops and accumulators varies among plant combinations. Rhizosphere interaction may mediate Cd uptake by intercropped plants, but the mechanism is unclear. Thus, in the present study, we aimed to examine the effect of rhizosphere interaction on Cd uptake by intercropping rice (Oryza sativa L.) with mugwort (Artemisia argyi Levl. et Vant.) in Cd-contaminated paddy soil. We grew O. sativa and A. argyi in pots designed to allow different levels of interaction: complete root interaction (no barrier), partial root interaction (nylon mesh barrier), and no root interaction (plastic film barrier). Our results indicated that both complete and partial root interaction increased the shoot and root mass of A. argyi, but did not decrease the shoot, root, and grain mass of O. sativa. Interspecific root interaction significantly increased the Cd content in the shoots, roots, and grains of O. sativa and the shoots of A. argyi. Increased content of total organic acids in the rhizosphere, which increased the content of available Cd, was a possible mechanism of increased Cd uptake in both plants under interspecific root interaction. Our findings demonstrate that an intercropping system can extract more Cd from contaminated soil than a monocropping system of either A. argyi or O. sativa. However, the intercropping system did not facilitate safe crop production because it substantially increased grain Cd content in O. sativa.

Phytoavailability of cadmium in rice amended with organic materials and lime: Effects of rhizosphere chemical changes and cadmium sequestration in iron plaque.

Excessive Cd in rice grains produced with acidic paddy soil is receiving increasingly widespread attention because it endangers human health. Applying organic materials (OM) and lime (L) is a common technique used to reduce Cd concentration in grains (CdG). Nevertheless, the mechanism by which their simultaneous application affects the Cd phytoavailability in soilrice systems remains ambiguous. In the current study, we adopted a rhizobag pot culture test to explore the influences of single application of OM [rice straw (RS), milk vetch (MV)], L, and their co-utilization on Cd phytoavailability and the associated mechanisms. The results showed that the application of RS, MV, L, L + RS (LRS), and L + MV (LMV) significantly decreased CdG by 26.9%, 38.2%, 48.6%, 50.0%, and 53.0%, respectively. Fe plaque (IP) formation was not affected by these treatments; however, Cd sequestration in IP (CdIP) was significantly reduced. CdIP was significantly reduced by 18.3%, 23.6%, 43.8%, 33.1%, and 41.4%, after RS, MV, L, LRS, and LMV treatments, respectively. Additionally, available Cd concentrations in rhizospheric soil (RHS) were significantly reduced by 11.5%, 14.8%, 15.1%, and 18.4%, after MV, L, LRS, and LMV treatments, respectively. Cd availability in RHS was significantly influenced by pH, dissolved organic carbon concentration, and Zn, Fe, and Mn availability. The results of the structure equation mode showed that CdG was mainly affected by CdIP, followed by Cd availability and the pH of RHS. In conclusion, the reduction of CdG by OM, L, and their co-utilization was the results of their combined effects of reducing Cd availability in RHS, CdIP, and Cd uptake by the roots. This study emphasizes that the reduction of CdG is a result of the dual effects of reducing Cd availability in RHS and CdIP after amendments application. L application alone or in conjunction with OM is an efficient practice to reduce CdG in acidic Cd-contaminated paddy fields.

Key process for reducing grains arsenic by applying sulfate varies with irrigation mode: Dual effects of microbe-mediated arsenic transformation and iron plaque.

Sulfate affects the transformation of arsenic (As) in soil and its absorption by plant roots. However, the influence of sulfate and irrigation interactions on the mobility of As in the soil-rice system remains poorly understood. To address this gap, we conducted a pot experiment with varying sulfate levels and irrigation modes to examine their effects on rice As translocation, soil As forms, iron plaque formation, and microorganisms involved in As transformation. The addition of exogenous sulfate significantly reduced grain As levels by a maximum of 60.1%, 46.7%, and 70.5% under flooding (F), flooding-moist alternate (FM), moist (M) conditions, respectively. However, the changes in soil available As did not fully correspond to grains As content. Soil available As was only reduced by sulfate under the FM treatment, which limited grains As accumulation under this condition. The reduction in grains As content under F and M conditions was mainly attributed to sulfate-induced increases in soil pH, which in turn inhibited As translocation and promoted iron plaque formation. Additionally, both irrigation mode and sulfate fertilization independently or interactively influenced the abundance of Sulfuritalea, Koribacter, Geobacter, and Sulfuriferula, thereby affecting the As forms in soil through the Fe/S redox process. Specifically, under F and FM conditions, SO42--S inhibited Geobacter but stimulated Fe-oxidizing bacteria, possibly resulting in increased As bound to Fe/Mn oxides (As-F3). Under M condition, SO42--S levels regulated As adsorption and release through the participation of Fe/S cycle bacteria, specifically influencing the adsorbed As fraction (As-F2). Therefore, the addition of SO42--S hindered As translocation to grains by promoting As sequestration in the iron plaque and facilitating microbe-mediated As immobilization through the Fe/S cycle, which was dependent on soil moisture. These results can be used as a guide for sulfur fertilizer application under different soil moisture with the goal of minimizing rice grain As.

Foliar zinc reduced Cd accumulation in grains by inhibiting Cd mobility in the xylem and increasing Cd retention ability in roots1.

Cadmium (Cd) pollution endangers the safe utilization of paddy soils, and foliar zinc (Zn) can reduce the toxic effects of Cd. However, little is known about the effects of foliar Zn application on the transport and immobilization of Cd in key rice tissues and the physiological state of rice plants. A pot experiment was conducted to explore the effects of spraying 0.2% and 0.4% Zn (ZnSO4) during the early grain-filling stage on Cd transport in rice, photosynthesis, glutathione (GSH) levels, Cd concentrations in xylem sap, and the expression of Zn transporter genes. The results showed that grain Cd concentrations in the 0.2% Zn and 0.4% Zn treatments were 24% and 31% lower, respectively, than those of the control treatments at maturity. Compared with the control treatments, the 0.4% Zn treatment increased Cd by 60%, 69%, 23%, and 22% in husks, rachises, first internodes, and roots, respectively. Application of Zn reduced xylem Cd content by up to 26% and downregulated transporter genes (OSZIP12, OSZIP4, and OSZIP7a) in flag leaves. Foliar Zn increased Cd bioaccumulation in roots while decreasing Cd bioaccumulation in grains. Zn reduced GSH concentration in flag leaves and stems, inhibiting photosynthesis (intercellular CO2 concentration, transpiration rate). Taken together, foliar Zn can reduce the expression of Zn transporter genes and the mobility of Cd in the xylem, promoting the fixation of Cd in husks, rachises, first internodes, and roots, ultimately reducing Cd concentration in rice grains.

Milk vetch returning reduces rice grain Cd concentration in paddy fields: Roles of iron plaque and soil reducing-bacteria.

Milk vetch (MV, Astragalus sinicus L.) is used in agricultural production as a green manure; however, its impact on accumulation levels of heavy metals (e.g., Cd) in rice remains poorly understood. This study investigated the effects of MV on Cd accumulation in rice, iron plaque formation, soil properties, and the soil microbial community structure through field experiments. The results showed that MV reduced Cd concentration in the roots, stem, leaves, and grains by 33%, 60%, 71%, and 49%, respectively. Chemical fertilizer and MV treatment promoted iron plaque formation, and MV considerably increased the Fe/Mn ratio in the iron plaque. More importantly, MV inhibited Cd transportation from the root iron plaque to the root by 74%. The concentrations of CaCl2-extractable Cd, available phosphorus, and available potassium, as well as the cation exchange capacity and urease activity, were significantly reduced in the MV treatment. Furthermore, 16 S rDNA high-throughput sequencing results of the soil microbial community structure showed that compared with the control, MV increased the soil microbial richness, increased the relative abundance of anaerobic microorganisms, and significantly increased the relative abundance of Thermodesulfovibrio and Geobacter at the genus level. The increase in anaerobic microbial abundance was closely related to the decrease in CaCl2-extractable Cd concentration. The application of MV promoted the formation of iron plaque, inhibited the transport of Cd, increased the abundance of anaerobic microorganisms, decreased the CaCl2-extractable Cd concentration, and reduced the Cd concentration in rice grain.

[Effects of Iron Intensity-regulated Root Microbial Community Structure and Function on Cadmium Accumulation in Rice].

Exploring the effects of exogenous iron (Fe) on cadmium (Cd) in rice is of great significance for ensuring food security. The accumulation of Cd and the changes in the microbial community structure in rice roots under three Fe concentrations (5, 50, and 500 µmol·L-1 EDTA-Na2Fe) were studied through a hydroponic experiment. The results showed that the increase in the environmental Fe concentration promoted the formation of iron plaque on the rice roots, and both Fe-deficiency and Fe-sufficiency would enhance the adsorption and fixation of Cd on the root surface. Compared with that of normal Fe levels (50 µmol·L-1), Fe deficiency increased Cd accumulation in rice roots and shoots by 49.76% and 15.68%, respectively. Although Fe sufficiency also increased Cd accumulation in the roots by 18.39%, the Cd concentration in shoots was significantly reduced by 35.95% compared with that of the normal Fe. 16S rRNA high-throughput sequencing was used to determine the root microbial community structure, and through PCA, LEfSe, and RDA analysis, it was found that compared with normal Fe, an Fe-deficient environment reduced the abundance and uniformity of root microbes. Proteobacteria and Bacteroidetes at the phylum level were the dominant flora, Fe deficiency inhibited the increase in the relative abundance of Bacteroidetes, and high-concentration Fe reduced the relative abundance of Proteobacteria. At the genus level, the relative abundance of functional microorganisms Ensifer, Rhodopila, Bdellovibrio, and Dyella were different under different Fe environments, which may have affected the absorption and accumulation of Cd by rice by affecting the formation of Fe plaque on the root and other biochemical processes. In addition, the effect of an Fe-deficient environment on microbial functions was higher than that of the Fe sufficient environment. This study investigated the changes in the rice root microbial community structure and the ability of rice to absorb and transport Cd under different Fe environments, which provided a theoretical basis and an important reference for the inhibition of Fe on Cd accumulation in rice in Cd-polluted paddy soil in southern China.

Fe fortification limits rice Cd accumulation by promoting root cell wall chelation and reducing the mobility of Cd in xylem.

Fe biofortification and Cd mitigation in rice is essential for human health, thus the effects of fertilization with Fe on Cd uptake and distribution in rice need to be comprehensively studied. In this study, we investigated the roles of root iron (Fe)/manganese (Mn) plaques, root cell wall, organic acid, and expressions of Cd-transport related genes in restricting Cd uptake and translocation. The rice plants were exposed to 1 µM CdCl2 with or without the addition of three doses of Fe at 5, 50, and 500 µM EDTA-Na2Fe. The results showed that increasing supply of Fe remarkably reduced Cd accumulation in the shoots, mainly because of inhibited translocation of Cd from roots to shoots. As compared to 5 µM Fe treatment, 500 µM Fe significantly increased the ionic soluble pectin (ISP) content and decreased citric acid (CA) in the roots, thereby providing more Cd-binding sites in the cell wall of roots and reducing the mobility of Cd in xylem. Plant Fe status-mediated CA act as the main chelator for Cd mobilization, rather than through decreasing the pH. However, the plants supplied with low Fe or excess Fe facilitated the uptake of Cd in rice roots, as low Fe up-regulated the expression of Cd-transport related genes and excess Fe enhanced Cd enrichment on the root by iron plaque. Importantly, soil fertilization with Fe strongly reduced Cd accumulation in rice grain. Thus, optimizing the soil environmental Fe could effectively reduce Cd accumulation in the shoots by immobilizing Cd in the roots.

Influence of straw-derived humic acid-like substance on the availability of Cd/As in paddy soil and their accumulation in rice grain.

Humic acid amendments have been widely advocated for the remediation of heavy metal-contaminated soil. However, the impacts of straw-derived humic acid-like substances on the remediation of cadmium (Cd) and arsenic (As) co-contaminated paddy soil remain unclear and the potential mechanism required clarification. In this study, we employed a pot experiment and chose a straw-derived humic acid-like substance (BFA) as the amendment with four doses to investigate how BFA affects the availability of Cd and As in soil and their accumulation in rice. The results showed that grain Cd decreased by 25.65-36.03%, while there was no significant change in total As (TAs) with the addition of BFA. The contents of DCB-Fe, DCB-As and DCB-Cd on the root surface decreased by 6.07-40.54% during the whole growth stage. The addition of BFA significantly decreased the pe + pH and enhanced the transformation of crystalline iron oxides (Fed) into amorphous forms (Feo) in the soil. The CaCl2-extractable Cd decreased and the KH2PO4-extractable As increased with the decrease in pe + pH and Fed and the relative increase in Feo. The correlation analysis showed that the decrease in availability of Cd and translocation factor of Cd effectively decreased the grain Cd and the decrease in DCB-Cd may also contribute to decreasing the uptake of Cd by rice. However, the increase in As of roots and shoots might play key roles in restricting the transport of As to rice grains. Consequently, the addition of BFA could effectively reduce the Cd accumulation in rice under flooding conditions, while no risk of As accumulation in rice grain was observed. The present work provides a new perspective for the application of straw-derived humic acid-like substances as amendments on Cd-As co-contaminated soils, which should be advocated as an eco-friendly, economical and effective soil amendment in the future.

CF1 reduces grain-cadmium levels in rice (Oryza sativa).

Rice (Oryza sativa) is a leading source of dietary cadmium (Cd), a non-essential heavy metal that poses a serious threat to human health. There are significant variations in grain-Cd levels in natural rice populations, which make the breeding of low-Cd rice a cost-effective way to mitigate grain-Cd accumulation. However, the genetic factors that regulate grain-Cd accumulation have yet to be fully established, thereby hindering the development of low-Cd varieties. Here, we reported a low-Cd quantitative trait locus, CF1, that has the potential to reduce Cd accumulation in rice grains. CF1 is allelic to the metal transporter OsYSL2, which transports Fe from the roots to the shoots. However, it is incapable of binding Cd, and thus, reduces grain-Cd levels indirectly rather than directly in the form of upward delivery. Further analysis showed that high expression levels of CF1 improve Fe nutrition in the shoots, subsequently inhibiting Cd uptake by systemically inhibiting expression of the main Cd uptake gene OsNramp5 in the roots. Compared with the CF1 allele from '02428' (CF102428 ), higher expression levels of CF1 from 'TQ' (CF1TQ ) increased the Fe contents and decreased Cd levels in rice grains. In natural rice populations, CF1TQ was found to be a minor allele, while CF102428 is present in most japonica rice, suggesting that CF1TQ could be widely integrated into the japonica rice genome to generate low-Cd varieties. Overall, these results broaden our mechanistic understanding of the natural variation in grain-Cd accumulation, supporting marker-assisted selection of low-Cd rice.

Nitrogen application practices to reduce cadmium concentration in rice (Oryza sativa L.) grains.

Cadmium (Cd) pollution in paddy soils creates challenges in rice grain production, thereby threatening food security. The effectiveness of different base-tillering-panicle urea application ratios and the combined basal application of urea and Chinese milk vetch (CMV, Astragalus sinicus L.) in minimizing Cd accumulation in rice grains was explored in a Cd-contaminated acidic soil via a field experiment. The results indicated that under similar nitrogen (N) application rates, an appropriate amount of urea applied at the panicle stage or the combined basal application of urea and CMV decreased Cd absorption by rice roots and its accumulation in rice grains, as compared with that of conventional N application (control). Furthermore, under a 3:4:3 base-tillering-panicle urea application ratio or under a high basal application of CMV (37,500 kg hm-2), Cd concentrations in brown rice were significantly lower (40.7% and 34.1%, respectively) than that of control. Cadmium transport coefficient from root to straw was significantly higher than that of control when an appropriate amount of urea was applied at the panicle stage or when urea and CMV were applied basally, whereas the Cd transport coefficient from straw to brown rice was relatively lower. Moreover, soil pH, or the CEC and CaCl2-Cd concentrations under different N fertilizer treatment was not significantly different. However, the rice grain yield increased by 29.4% with basal application of a high CMV amount compared with that of control. An appropriate amount of urea applied at the panicle stage or the combined basal application of urea and CMV decreased Cd absorption by rice roots and inhibited its transport from straw to brown rice, thus reducing Cd concentration in brown rice. Therefore, combined with the key phase of Cd accumulation in rice, a reasonable urea application ratio or a basal application of high CMV amounts could effectively reduce Cd concentration in brown rice.

Cadmium long-term immobilization by biochar and potential risks in soils with different pH under combined aging.

Cd long-term immobilization by biochar and potential risk in soils with different pH were quantified under a combined artificial aging, which simulated five years of aging in the field based on local climate. Two biochars (original and KMnO4-modified) and five soils with different pH were tested, and an improved three-layer mesh method was employed in this study. Five aging cycles were carried out (Cycle 1-Cycle 5), and each aging cycle quantitatively simulated 1 year of natural aging. As the aging time increased, Cd leaching loss in all soils gradually increased from Cycle 1 to Cycle 5; for relatively stable Cd fraction, it decreased firstly and then stabilized in acidic and neutral soils (S1-S4), while it decreased firstly and then increased in alkaline soil (S5). Biochars significantly promoted Cd immobilization in strongly acidic soil (S1) by increasing relatively stable fractions and decreasing leaching loss. For weakly acidic and neutral soils (S2-S4), although biochars still had positive effects, the immobilization effects were weakened to certain extents compared with S1. The percentage of Cd leaching loss decreased by 19.12% in strongly acidic soil (S1) and by 1.12-11.35% in weakly acidic and neutral soils (S2-S4) after modified biochar treatment. For alkaline soil (S5), the application of biochars had negative effects on Cd immobilization by decreasing relatively stable fractions and increasing leaching loss, and posed risks to the environment. For strongly acidic soil (S1) and weakly acidic and neutral soils (S2-S4), the percentages of relatively stable fractions increased from 6.09-19.93% to 24.98-36.70% after modified biochar treatment. However, for alkaline soil, the percentage of relatively stable fractions decreased from 55.27% to 53.93% after biochar treatment. The more acidic the soil, the more effective the Cd immobilization by biochar. Biochars with high pH level are not suitable for the remediation of alkaline Cd contaminated soil.

Sulfur fertilization integrated with soil redox conditions reduces Cd accumulation in rice through microbial induced Cd immobilization.

Sulfate and water management can be respectively applied to control Cd accumulation in rice, but the interaction mechanisms remain unclear. Three water management coupled with five sulfate application concentrations were employed to investigate rice Cd uptake. Results showed there was a significant interaction between sulfate application and soil redox state, and the highest sulfate treatments reduced rice grain Cd by 63.2, 53.5, and 59.4% under the flooding, flooding-moist alternate (FM), and moist irrigation (M) conditions, respectively. It could be explained by the reduction in rhizosphere soil available Cd and lower transport coefficient from root to aboveground. The Desulfovibrio was demonstrated to participate in CdS precipitation, and its abundance was promoted by sulfate especially under flooding. Additionaly, sulfate application facilitated Cd bounded to FeMn oxides, as rhizosphere soil pH raising under flooding. Under FM and M treatments, sulfate application reduced the abundance of Fe-reducing bacteria Geobacter, and correspondingly reduced Fe and Cd availability in rhizosphere soil. Summarily, Cd transfer from soil to rice can be reduced by applying sulfate fertilizer; which is favored by higher soil moisture because of the higher abundance of Desulfovibrio and lower abundance of Geobacter.

Combined exogenous selenium and biochemical fulvic acid reduce Cd accumulation in rice.

Paddy soil Cd contamination and the related accumulation risk in rice grains have attracted global attention. The application of selenium and humic substances is considered to be a cost-effective Cd mitigation measure. However, the effect of a combined application of the two materials remains unclear. Therefore, a 2-season pot experiment was conducted, wherein sodium selenite (Se) and biochemical fulvic acid (BFA) were applied alone and together. Paddy soils with two levels of Cd contamination were used. The results indicate that Se application alone considerably decreased the rice grain Cd content by 36.1-48.7% compared to the control rice grain Cd concentration, which was above the food safety limit (0.2 mg kg-1). Although the application of BFA alone decreased the soil pH, it also increased the soil CaCl2 extractable Cd content by 0.2 to 19.3% and had a limited effect on Cd in the rice grains. The combined application of Se and BFA did not affect the soil pH or the CaCl2 extractable Cd, and more effectively reduced the Cd contents of the rice grains by 50.2 to 57.1%, except for the control rice grain Cd content, which was below the limit. The combined application of Se and BFA also inhibited Se accumulation in rice grains, maintaining the Se content at a safe level (0.33-0.58 mg kg-1) compared to Se application alone. The effects of reducing the Cd content of rice grains while safely increasing their Se contents could persist for at least two seasons. Therefore, the combined application of Se and BFA should be recommended to mitigate Cd contamination risks in Cd-contaminated paddy soil.

Effect of humic and calcareous substance amendments on the availability of cadmium in paddy soil and its accumulation in rice.

Humic substances (HS) are widely known as important components in soil and significantly affect the mobility of metals due to their large surface area and abundant organic functional groups. Calcareous substances (CSs) are also commonly used as robust and cost-effective amendments for increasing the pH of acidic soils and decreasing the mobility of metals in soils. In this study, we developed a new remediation scheme for cadmium (Cd)-contaminated soil remediation by coupling HS and CS. The results showed that regardless of the addition of fulvic acid (FA), all the CS-containing treatments significantly increased the soil pH by 0.32-0.60, and the concentration of bioavailable Cd decreased in the moderately (field experiment soil, maximum 62%) and highly (pot experiment soil, maximum 57%) Cd-contaminated soils. The Cd content in rice (Oryza sativa L.) tissues significantly decreased after all the treatments. The bioaccumulation factors (BAFs) decreased by over 50% in the roots, stems, leaves and husks in all treatments, while the translocation factors (TFs) only significantly decreased in the highly contaminated soil. Among all treatments, the two HS+CS treatments (FA+CaCO3 and FA+CaO) had the greatest effect on decreasing the concentration of bioavailable Cd in soil and Cd in brown rice grains. The suggested mechanism for the effectiveness of coupled HS and CS was that CS first mitigated the pH and precipitated Cd, followed by a complexation effect between HS and Cd. Although the Cd in rice grains in both cases was higher than the standard limit, HS+CS remediation can be advocated as a robust, simple and cost-effective scheme for Cd remediation if the additive dose is slightly increased, as this approach can simultaneously improve the pH of acidic soil and adsorb Cd in soil.

Robust identification of low-Cd rice varieties by boosting the genotypic effect of grain Cd accumulation in combination with marker-assisted selection.

Rice (Oryza sativa L.), a staple for half of the world's population, usually accumulates high levels of cadmium (Cd) in the grain when planted in the Cd-contaminated paddy fields. Genetic improvements using natural variation of grain-Cd accumulation is the most cost-effective way to mitigate the risk of excess Cd accumulation. However, as a complex trait, grain-Cd accumulation is susceptible to environmental variation, which challenges to characterize the genetic nature and subsequently the stable performance of grain-Cd accumulation. To boost the genetic effect on grain-Cd performance, we established an approach of normalization using the comparative grain-Cd value (CCd) following a contrasting field design. Identification of the genetic locus responsible for CCd variation help us develop a low-grain-Cd variety de novo, named 'Lushansimiao', which had lower grain-Cd levels in a large-scale field test and can produce Cd-safe rice following prolonged irrigations in the field with intermediate levels of Cd pollution. Combined CCd evaluating and low-Cd allelic genotyping, another six varieties were also identified as low-grain-Cd rice. Our study paves the way to efficiently quantify the genetic nature of grain-Cd accumulation in rice, and the stable low-Cd rice varieties will help to mitigate the risk of excess Cd accumulation in rice.

Cadmium and arsenic availability in soil under submerged incubation: The influence of humic substances on iron speciation.

Humic substances (HSs), as electron shuttles, are associated with iron oxide transformation, yet the manner by which HSs affect Cd/As availabilities during this process under anaerobic conditions remains unclear. Two HSs (humic sodium, HA-Na, and biochemical fulvic acid, BFA) were applied at 0, 1, 2, and 4 gCkg-1 in a submerged incubation experiment. The dissolved, extractable and fractions of Cd/As and different iron oxides in soils were monitored. The addition of both HA-Na and BFA decreased the CaCl2-extractable Cd by 12.66-93.13%, and increased the KH2PO4-extractable As by 18.81-71.38% on the 60th day of incubation. The soil Eh and crystalline iron oxides (Fed) decreased, while amorphous iron oxides (Feo) and dissolved As increased after addition of both HSs. However, the two HSs had opposite effects on soil pH and dissolved Cd at the end of the incubation. HA-Na immobilized 19.47-85.99% more available Cd than did BFA over the incubation, although the extent of immobilization was similar with the maximum application rate on the 60th day. BFA mobilized 5.22-26.12% more available As than did HA-Na. XPS data showed that FeOOH decreased while the FeO component increased over the incubation. Correlation analysis and SEM showed that the reduction in the soil Eh and Fed and relative increase in Feo increased the available Cd, while decreased the available As. Consequently, the addition of HA-Na and BFA, particularly combined with flooding irrigation management, could effectively reduce the available Cd in Cd-contaminated soil. However, this method should be used with caution in As-contaminated soil.

Meta-analysis of the effects of liming on soil pH and cadmium accumulation in crops.

Increasing cadmium (Cd) contamination in agricultural fields has resulted in a higher risk of Cd accumulation in the food chain. Lime addition can mitigate soil acidification and reduce Cd accumulation in crops cultured in Cd-contaminated soil. To determine key factors controlling the outcomes of liming in reducing Cd accumulation and enhancing soil pH, we performed a meta-analysis using previously published data from field and pot experiments. The results indicated that the liming showed positive effect sizes on the soil pH but negative effect sizes on Cd accumulation in crops, indicating the addition of different lime materials could enhance soil pH and reduce Cd accumulation in crops. The effect sizes of liming on soil pH under pot experimental conditions were higher than that under field experimental conditions, however, the effect sizes of application types and amount of limes on soil pH did not significantly differ between their individual different levels. Under a low background value of soil pH, SOM, CEC and clay, the addition of limes showed a significantly higher effect size on soil pH when compared to their individual higher soil background value, suggesting that the lower background values of soil pH, SOM, CEC and clay might facilitate the outcomes of liming to enhance soil pH. The experiment patterns, crop types and lime application amounts showed a limit effect on the outcomes of liming to reduce the shoot and grain Cd concentrations in crops. The lime types only showed a significant effect size on the shoot Cd accumulation but not on the grain Cd accumulation, in which the CaCO3 had the highest effect size (absolute value, the same below) followed by Ca(OH)2 and CaO. The low soil background values of total Cd concentration and CEC content, but a high soil SOM background content might facilitate the outcomes of liming to reduce the shoot Cd concentration in crops. However, only the background value of soil clay content showed a significantly negative effect size on the grain Cd accumulation, where a high soil clay content had a higher effect size than a low soil clay content. These findings provide useful knowledge about the effects of experiment patterns, crop types, soil conditions, lime types and lime addition amounts on the efficiency of liming in enhancing soil pH and decrease crop Cd concentration.

Influences of different source microplastics with different particle sizes and application rates on soil properties and growth of Chinese cabbage (Brassica chinensis L.).

The potentially negative effects of microplastics (MP) on agroecosystems have raised worldwide concerns. However, little is known about the negative effects of MP exposure on the soil-plant system. To fill up this knowledge gap, a pot experiment was set up, and two different MP types [high density polyethylene (HDPE) and general purpose polystyrene (GPPS)] were used, which had four particle sizes (<25, 25-48, 48-150, and 150-850 µm) at four application rates (2.5, 5, 10, and 20 g MP kg-1 soil). Some soil properties and the growth of Chinese cabbage (Brassica chinensis L.) were monitored. The results showed that (1) MP application with high application rates and relatively small particle sizes significantly enhanced the soil urease activity, which accompanied with enhanced soil pH and decreased soil available concentrations of phosphorus and potassium in some cases. (2) The exposure of MP did not significantly affect the activity of soil catalase regardless of their application rates and sizes. MP with different application rates and small sizes significantly reduced the soil sucrase activity, but the largest size of MP enhanced the activity of soil sucrase. (3) GPPS at 10-20 g kg-1 or with the sizes of <25 and 48-150 µm significantly reduced the fresh weight of Chinese cabbage, but the addition of HDPE had no remarkable effects on the fresh weight regarding of its application rates or sizes. (4) MP with high application rates and large sizes enhanced but small sizes of MP reduced the leaf soluble sugar concentration. The increasing application rates of MP and small size HDPE significantly reduced the starch concentration in the leaves of Chinese cabbage, however, the different sizes of GPPS showed limited effects on the leaf starch. The addition of MP with increasing application rates and different sizes always reduced the concentration of leaf chlorophyll. These parameters regarding to plant and soil could be used to assess the risks of MP pollution in the soil-plant systems. We found that the risks resulting from MP pollution were MP type-dependent and particle size-dependent. These findings indicate that overaccumulation of MP in the agriculture may possess an ecology risk and will negatively affect the agricultural sustainability and the food safety.

[Effects of Water Management on Cadmium Accumulation by Rice (Oryza sativa L.) Growing in Typical Paddy Soil].

In order to explore the effects of water management on the Cd accumulation of rice in paddy soils with different parent materials, a pot experiment with three paddy soils with different parent materials from Hunan Province (granite sandy soil, plate shale soil, and purple sandy shale soil) with different water management treatments ï¼»flooding and alternate wetting and drying (AWD)ï¼½ was performed. The soil pH, DTPA-Cd, Fe plaque in the rice roots, and heavy metal concentration in the rice were determined. The results showed that the soil pH of the three paddy soils under the flooding treatment was increased by 0.17-1.33 units. During the filling and maturity periods, compared with that under AWD, the DTPA-Cd concentration in the three paddy soils was reduced by 14.39%-36.56% under the flooding treatment, but the DTPA-Fe concentration was increased by 35.35%-347.25%. In the three growth stages, the Cd and Mn concentrations in the Fe plaque (except for DCB-Fe) were in the order of tillering stage < filling stage < mature stage. Compared with that under AWD, the brown rice Cd concentration in the three soils was reduced by 57.84%-93.79% under flooding treatment. The Cd accumulation in rice was reduced under flooding treatment by reducing the DTPA-Cd via increasing the soil pH and DTPA-Fe and by decreasing the formation of Fe plaque. According to the results of the correlation and SEM analysis, the soil pH and DCB-Cd were the main factors affecting the Cd accumulation in rice grains, although the changes in the DTPA-Cd and DTPA-Fe also impacted the Cd in rice grains. In summary, our study demonstrated that water management had a significant impact on the Cd content in rice, and there were significant differences among the three paddy soils with different parent materials. In conclusion, the Cd content in rice grains was affected by the soil parent material, soil physicochemical properties, and Fe plaque on the surface of the rice roots. The granite sandy soil and plate shale soil with different water management treatments had significant impacts on the contents of heavy metals in rice. Continuous flooding is a valuable strategy for improving soil acidity and alkalinity and minimizing soil available Cd, but the soil parent materials must be considered.