[Cadmium Phytoremediation Effect of Sweet Sorghum Assisted with Citric Acid on Typical Parent Soil in Southern China].

Sweet sorghum has a large biomass and strong cadmium (Cd) absorption capacity, which has the potential for phytoremediation of Cd-contaminated soil. In order to study the Cd phytoremediation effect of sweet sorghum assisted with citric acid on the typical parent materials in southern China, a field experiment was carried out in two typical parent material farmland areas (neutral purple mud field and jute sand mud field) with Cd pollution in Hunan Province. The results showed that:① Citric acid had no inhibitory effect on the growth of sweet sorghum. After the application of citric acid, the aboveground biomass of sweet sorghum at the maturity stage increased by 10.1%-24.7%. ② Both sweet sorghum planting and citric acid application reduced the soil pH value, and the application of citric acid further reduced the soil pH value at each growth stage of sweet sorghum; this decrease was greater in the neutral purple mud field, which decreased by 0.24-0.72 units. ③ Both sweet sorghum planting and citric acid application reduced the total amount of soil Cd, and the decreases in the neutral purple mud field and jute sand mud field were 23.8%-52.2% and 17.1%-31.8%, respectively. The acid-extractable percentage of soil Cd in both places increased by 38.6%-147.7% and 4.8%-22.7%, respectively. ④ The application of citric acid could significantly increase the Cd content in various tissues of sweet sorghum. The Cd content in the aboveground part of the plant in the neutral purple mud field was higher than that in the jute sand mud field, and the Cd content in stems and leaves was 0.25-1.90 mg·kg-1 and 0.21-0.64 mg·kg-1, respectively. ⑤ After applying citric acid, the Cd extraction amount of sweet sorghum in neutral purple mud soil in the mature stage reached 47.56 g·hm-2. In summary, citric acid could enhance the efficiency of sweet sorghum in the phytoremediation of Cd-contaminated soil, and the effect was better in neutral purple mud fields. This technology has the potential for remediation coupled with agro-production for heavy metal-contaminated farmland.

[Effect of Silica Fertilizer(Husk Ash) to Improve Soil Quality and Reduce Cd and As Accumulation in Rice].

A rice pot experiment was conducted to identify the effect of silica fertilizer prepared from husk ash on the soil bioavailability of cadmium (Cd) and arsenic (As), enzyme activities, microbial community structure, and heavy metal content in brown rice at different growth stages. The results showed that the application of 0.1%-1.0% silica fertilizer-husk ash increased the pH value of soil by 0.04-0.24 units and the content of soil available silicon by 44.2%-97.5%. It also decreased the content of available Cd and available As by 16.2%-21.4% and 16.0%-24.9%, respectively. With the increase in application amount, the soil enzyme activities increased at all growth stages, and the sucrase activity and the dehydrogenase activity significantly increased by 6.3%-145.7% and 6.7%-224.1%, respectively. The analysis of the soil microbial community composition structure at mature stages showed that the application of silica fertilizer-husk ash had no effect on microbial α-diversity, but it had a significant effect on microbial ß-diversity and then promoted microbial growth and maintained the stability of the community structure. With the increase in application amount, the contents of Cd in brown rice decreased by 29.3%-89.7%, and the contents of total As and inorganic As in brown rice decreased by 7.8%-42.3% and 17.2%-44.5%, respectively. Under the application of 0.5% and 1.0% silica fertilizer-husk ash, the Cd contents in brown rice were lower than 0.2 mg·kg-1, and the inorganic As contents in brown rice were lower than 0.35 mg·kg-1. In conclusion, the silica fertilizer-husk ash can improve soil quality and reduce the contents of Cd and As in brown rice, and it is eco-friendly and can be used to remedy the paddy soil contaminated with Cd and As.

[Effects of the Application of Irrigation Water Containing Zn at the Key Growth Period on the Uptake and Transport of Cd in Rice].

In this study， a field experiment was conducted to examine the effects of the application of irrigation water containing Zn at the key growth period （booting stage and filling stage） on exchangeable Cd content in the soil， Cd concentration in pore water， and Cd uptake and transport in rice in a Cd-contaminated paddy field in Liuyang City， Hunan Province. The results indicated that： ① the application of irrigation water containing Zn during the key growth period could inhibit the releasing process of exchangeable Cd from the soil into pore water. Compared with that in the control， the content of exchangeable Cd in soil was slightly changed， but the concentration of Cd in soil pore water at the mature stage was significantly reduced by 16.7%-57.6%. ② The application of irrigation water containing Zn at the key growth period could significantly reduce the Cd content in various parts of rice. Cd contents in root， stem， and brown rice with the application of irrigation water containing 20 mg·L-1 Zn before the booting and the filling stage （BF1） were significantly decreased by 56.0%， 83.8%， and 85.2%， respectively. ③ Compared with the application of 100 mg·L-1 irrigation water containing Zn， the application of 20 mg·L-1 irrigation water containing Zn significantly reduced the uptake and transport of Cd in rice， and the translocation factor （TF） of Cd from rice roots to stems was also significantly reduced by 12.5%-56.3%， with the B1 and BF1 treatments reaching significant levels. These results suggested that the application of irrigation water containing Zn could significantly reduce the uptake and accumulation of Cd in rice， and the application of 20 mg·L-1 irrigation water containing Zn before the booting and filling stage could effectively realize the safe production of Cd-contaminated paddy fields.

Physiological tolerance of black locust (Robinia pseudoacacia L.) and changes of rhizospheric bacterial communities in response to Cd and Pb in the contaminated soil.

Woody plants possess great potential for phytoremediation of heavy metal-contaminated soil. A pot trial was conducted to study growth, physiological response, and Cd and Pb uptake and distribution in black locust (Robinia pseudoacacia L.), as well as the rhizosphere bacterial communities in Cd and Pb co-contaminated soil. The results showed that R. pseudoacacia L. had strong physiological regulation ability in response to Cd and Pb stress in contaminated soil. The total chlorophyll, malondialdehyde (MDA), soluble protein, and sulfhydryl contents, as well as antioxidant enzymes (superoxide dismutase, peroxidase, catalase) activities in R. pseudoacacia L. leaves under the 40 mg·kg-1 Cd and 1000 mg·kg-1 Pb co-contaminated soil were slightly altered. Cd uptake in R. pseudoacacia L. roots and stems increased, while the Pb content in the shoots of R. pseudoacacia L. under the combined Cd and Pb treatments decreased in relative to that in the single Pb treatments. The bacterial α-diversity indices (e.g., Sobs, Shannon, Simpson, Ace, and Chao) of R. pseudoacacia L. rhizosphere soil under Cd and Pb stress were changed slightly relative to the CK treatment. However, Cd and Pb stress could significantly (p < 0.05) alter the rhizosphere soil microbial communities. According to heat map and LEfSe (Linear discriminant analysis Effect Size) analysis, Bacillus, Sphingomonas, Terrabacter, Roseiflexaceae, Paenibacillus, and Myxococcaceae at the genus level were notably (p < 0.05) accumulated in the Cd- and/or Pb-contaminated soil. Furthermore, the MDA content was notably (p < 0.05) negatively correlated with the relative abundances of Isosphaeraceae, Gaiellales, and Gemmatimonas. The total biomass of R. pseudoacacia L. was positively (p < 0.05) correlated with the relative abundances of Xanthobacteraceae and Vicinamibacreraceae. Network analysis showed that Cd and Pb combined stress might enhance the modularization of bacterial networks in the R. pseudoacacia L. rhizosphere soil. Thus, the assembly of the soil bacterial communities in R. pseudoacacia L. rhizosphere may improve the tolerance of plants in response to Cd and/or Pb stress.

[Effects of Straw Removal Measure on Soil Cd Bioavailability and Rice Cd Accumulation].

A field experiment was conducted in a lightly Cd-contaminated rice field in Ningxiang City, Hunan Province, to study the effects of straw removal measures on the soil Cd bioavailability and rice Cd accumulation. The results showed that:① two consecutive seasons of straw removal measures (T1-T4 treatments) effectively increased soil pH by 0.04-0.58 units, reduced soil organic matter by 0.68%-25.87%, and reduced the Cd content of rhizosphere soil by 3.76%-12.78%. ② The proportions of Cd in the acid-extractable fraction and oxidizable fraction decreased, and the proportion of Cd in the residual fraction increased. Furthermore, straw removal measures significantly reduced the bioavailability of Cd in rhizosphere soil, and the Cd contents in TCLP, DTPA, and CaCl2 extracts all significantly decreased compared with those in CK. ③ The straw removal measure could significantly reduce the content of DOC and Cd in soil pore water; and the contents of Cd in soil pore water decreased by 4.54%-40.00% and 2.75%-67.34% under the straw removal measure (T1-T4) for two consecutive seasons, respectively, indicating that DOC was one of the key factors affecting the content of Cd in soil pore water. ④ Two consecutive straw removal measures (T1-T4) reduced the accumulation of Cd in different rice tissues, among which, under the treatment of all straw and root removal (T4), the Cd contents of brown rice in late rice planting in 2020 and early rice planting in 2021 decreased by 18.52% and 39.69%, respectively. Therefore, full or partial removal of straw in Cd-contaminated rice fields is a powerful measure to reduce the risk of exceeding Cd levels in brown rice.

Simultaneous alleviation of Cd availability in contaminated soil and accumulation in rice (Oryza sativa L.) by Fe-Mn oxide-modified biochar.

Fe-Mn oxide-modified biochar (BC-FM) was used to remediate Cd-contaminated soil and mitigate Cd accumulation in rice. The roles of Fe and Mn in soil Cd immobilization and in controlling Cd uptake by rice were investigated via X-ray photoelectron spectroscopy (XPS) characterization and chemical analysis. Fe and Mn loaded on BC-FM increased the removal efficiencies of CaCl2 extractable Cd in soil and Cd in pore water compared to those in only biochar (BC)-treated soil, with maximum removal rates at 67.9 % and 77.8 %, respectively. The XPS results indicated that the redox reactions of the Fe-Mn oxides on BC-FM surface affected Cd immobilization in the soil. The Fe (II/III) components on BC-FM were primarily converted to Fe3O4 in the soil system, which may form stable complexes with Cd2+ (Fe-O-Cd) during the entire rice growth period, and Cd may be bound to MnO or Mn2O3 in the form of CdMn2O4. The excellent adsorption performance of BC-FM enhanced by Fe-Mn oxides reduced the available Cd in the soil and stimulated Fe and Mn transport in rice, thereby inhibiting Cd accumulation in the aerial parts of rice. Cd concentrations in brown rice under BC-FM treatments reached the national safety standard (0.2 mg/kg, GB2762-2017). And BC-FM significantly increased the biomass of brown rice with a maximum rate of 26.8 %. These findings suggest that BC-FM could be used as an efficient material for Cd-contaminated soil remediation, and Fe-Mn plays important role in immobilizing Cd in soil and reducing Cd transport in rice.

[Effect of Citric Acid and Mowing on Enhancing the Remediation of Cadmium Contaminated Soil by Napier Grass (Pennisetum purpureum Schum)].

A pot experiment was conducted to investigate the effects of citric acid application and mowing frequency on the remediation of cadmium (Cd) contaminated soil by napier grass (Pennisetum purpureum Schum). Three levels of citric acid were divided into three applications of 1.25, 2.5, and 5 mmol·kg-1. The mowing frequency of the plants was divided into no mowing, one mowing, and two mowing treatments. The results showed that:① 1.25 mmol·kg-1 citric acid increased the biomass of the upper part of the plant by 39.11% with one mowing, and multiple mowing treatments and high citric acid application were not beneficial to the biomass increase. ② Both citric acid application and mowing had the effect of increasing the Cd content in stems and leaves, and Cd content in stems harvested in the last mown crop was larger and increased by approximately six times under the 5 mmol·kg-1 citric acid application. ③ Citric acid application and mowing reduced the rhizosphere soil pH and organic matter and also reduced the total soil Cd content and TCLP-Cd content by a maximum of 14.29% and 10.17%, respectively. ④ Under the 1.25 mmol·kg-1citric acid application and one mowing treatment (L1), the best Cd extraction by Napier grass was achieved with 6.95 mg·plant-1 of above-ground parts, accounting for 9.38% of the total Cd content in the potted test soil. Therefore, the L1 treatment can be considered to improve the remediation efficiency when using napier grass to remediate Cd-contaminated soil in the future.

Adsorption Characteristics and Mechanisms of Fe-Mn Oxide Modified Biochar for Pb(II) in Wastewater.

This study prepared iron-manganese oxide-modified biochar (FM-BC) by impregnating rice straw biochar (BC) with a mixed solution of ferric nitrate and potassium permanganate. The effects of pH, FM-BC dosage, interference of coexisting ions, adsorption time, incipient Pb(II) concentration, and temperature on the adsorption of Pb(II) by FM-BC were investigated. Moreover, the Pb(II) adsorption mechanism of FM-BC was analyzed using a series of characterization techniques. The results showed that the Fe-Mn oxide composite modification significantly promoted the physical and chemical functions of the biochar surface and the adsorption capacity of Pb(II). The specific surface area of FM-BC was 18.20 times larger than that of BC, and the maximum Pb(II) adsorption capacity reached 165.88 mg/g. Adsorption kinetic tests showed that the adsorption of Pb(II) by FM-BC was based on the pseudo-second-order kinetic model, which indicated that the adsorption process was mainly governed by chemical adsorption. The isothermal adsorption of Pb(II) by FM-BC conformed to the Langmuir model, indicating that the adsorption process was spontaneous and endothermic. Characterization analyses (Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy) showed that the adsorption mechanism of Pb(II) by FM-BC was mainly via electrostatic adsorption, chemical precipitation, complexation, ion exchange, and the transformation of Mn2O3 into MnO2. Therefore, FM-BC is a promising adsorbent for Pb(II) removal from wastewater.

[Effects of Phosphorus Sufficiency and Deficiency on Cadmium Uptake and Transportation by Rice].

Phosphorus (P) is an essential nutrient element for crop growth. The effects of P surplus or deficit on Cd absorption and transport in rice in Cd-polluted farmland is not clear. The effects of P deficiency and P sufficiency on Cd uptake, transport, and accumulation in rice under Cd stress were investigated by applying different levels of phosphorus (NaH2PO4) in a hydroponic experiment. The results showed that:① with the increase in ρ(P) (1.5-48.0 mg·L-1), the biomass in all parts of the rice plants had no obvious change, and the contents of photosynthetic pigment (chlorophyll a, chlorophyll b, and carotenoid) firstly ascended and then descended; high concentrations of P inhibited the synthesis of photosynthetic pigments. ② Under Cd stress, when the P was deficient (1.5-6.0 mg·L-1) or sufficient (12.0-48.0 mg·L-1), the Cd content in different parts of the rice increased with the increase in P addition level, and the maximum increase in Cd content in brown rice was 132.1% and 191.2%, respectively. ③ The P/Cd of brown rice showed a piecewise decreasing rule under P deficiency and P sufficiency, and the Cd content in brown rice was significantly negatively correlated with P/Cd (P<0.01). These results indicated that elevating phosphorus concentration when rice was under both the conditions of P deficiency and P sufficiency could promote the uptake and transport of Cd by rice roots under Cd stress, thus increasing the accumulation of Cd in aboveground parts and the risk of excessive Cd in rice.

Dynamic responses of soil enzymes at key growth stages in rice after the in situ remediation of paddy soil contaminated with cadmium and arsenic.

The practical application of in situ remediation techniques requires an understanding of the dynamic changes in soil enzyme activity as indicators of soil fertility and health. Experiments were carried out in paddy soils co-contaminated with cadmium (Cd) and arsenic (As) at low (L) and high (H) levels. A calcium and iron (CaFe)-based amendment (limestone + iron powder + silicon fertilizer + calcium­magnesium-phosphate fertilizer) was applied to the soil at concentrations of 0, 450, and 900 g·m-2 (labeled CK, T1, and T2, respectively), and sampling was conducted at the tillering (TS), booting (BS), filling (FS), and mature (MS) stages. In soil L, urease activity increased significantly by 15.8% under T1 treatment at the MS, catalase activity increased significantly under T2 treatment by 52.4% at the FS and 25.9% at the MS, and acid phosphatase activity increased significantly by 50.1%-65.9% at the TS. For soil H, urease activity increased by maximum values of 101.6% and 28.6% at the FS and MS, respectively. Catalase activity increased by 29.0% at the MS under T2 treatment, and acid phosphatase activity increased by maximum values of 40.5%, 16.0%, and 53.9% at the BS, FS, and MS, respectively. The results indicate that the changes in soil enzyme activity were mainly related to the rice growth stage, soil pH, and available Cd and As after the application of Ca-Fe-based amendment. Overall, at the FS and MS, the amendment increased the soil pH, soil enzyme activity, and cation exchange capacity and reduced the available Cd and As, which reduced the Cd and As contents in brown rice.

Enhancing Cd(II) adsorption on rice straw biochar by modification of iron and manganese oxides.

Metal oxide-modified biochar showed excellent adsorption performance in wastewater treatment. Iron nitrate and potassium permanganate were oxidative modifiers through which oxygen-containing groups and iron-manganese oxides could be introduced into biochar. In this study, iron-manganese (Fe-Mn) oxide-modified biochar (BC-FM) was synthesized using rice straw biochar, and the adsorption process, removal effect, and the mechanism of cadmium (Cd) adsorption on BC-FM in wastewater treatment were explored through batch adsorption experiments and characterization (SEM, BET, FTIR, XRD, and XPS). Adsorption kinetics showed that the maximum adsorption capacity of BC-FM for Cd(II) was 120.77 mg/g at 298 K, which was approximately 1.5-10 times the amount of adsorption capacity for Cd(II) by potassium-modified or manganese-modified biochar as mentioned in the literature. The Cd(II) adsorption of BC-FM was well fit by the pseudo-second-order adsorption and Langmuir models, and it was a spontaneous and endothermic process. Adsorption was mainly controlled via a chemical adsorption mechanism. Moreover, BC-FM could maintain a Cd removal rate of approximately 50% even when reused three times. Cd(II) capture by BC-FM was facilitated by coprecipitation, surface complexation, electrostatic attraction, and cation-π interaction. Additionally, the loaded Fe-Mn oxides also played an important role in the removal of Cd(II) by redox reaction and ion exchange in BC-FM. The results suggested that BC-FM could be used as an efficient adsorbent for treating Cd-contaminated wastewater.

Combined amendment improves soil health and Brown rice quality in paddy soils moderately and highly Co-contaminated with Cd and As.

In situ remediation technology applied aims to not only decrease cadmium (Cd) and arsenic (As) uptake by rice but also improve soil health and rice quality in contaminated paddy soils. Here the effects of a combined amendment, consisting of limestone, iron powder, silicon fertilizer, and calcium-magnesium-phosphate fertilizer, with three application rates (0, 450, and 900 g m-2) on soil health, rice root system, and brown rice quality were compared in moderately versus highly Cd and As co-contaminated paddy fields. After the amendment application, soil pH, cation exchange capacity, four kinds of soil enzyme activities increased (sucrase, urease, acid phosphatase, and catalase), and concentrations of leached Cd/As decreased, as measured by the DTPA (diethylene triamine pentaacetic acid) and TCLP (toxicity characteristic leaching procedure). Changes in the above soil indicators promoted soil health. In both fields, the dithionite-citrate-bicarbonate (DCB)-Fe and DCB-Mn concentration in iron plaque increased and root length became longer. Changes in the above root system indicators reduced the root system's absorption of Cd and As but increased that of nutrients. Under 900 g m-2 treatment, the Cd concentration in brown rice of two sites decreased by 55.8% and 28.9%, likewise inorganic As (iAs) decreased by 50.0% and 21.1%, whereas essential amino acids increased by 20.4% and 20.0%, respectively. Furthermore, the Cd and iAs concentrations in brown rice were <0.2 mg kg-1 (maximum contaminant level of Cd and iAs in the Chinese National Food Safety Standards GB2762-2017 for brown rice) under the 900 g m-2 in the moderately contaminated field. These results suggest the combined amendment can improve soil health and brown rice quality in the moderately and highly Cd- and As-co-contaminated paddy soils, offering potential eco-friendly and efficient remediation material for applications in such polluted paddy soils.

[Combined Effects of Soil Amendment and Zinc Fertilizer on Accumulation and Transportation of Cadmium in Soil-Rice System].

A field experiment was conducted in moderately and severely Cd contaminated paddy fields in Beishan Town, Changsha City, Hunan Province. This study examined the effects of LS amendment (limestone+sepiolite), in combination with soil application and foliar spraying of Zn fertilizer, on Cd uptake in early and late rice plants. The results showed that: ① the application of LS (2250 kg·hm-2 and 4500 kg·hm-2) significantly increased pH and CEC values in paddy soil during the early and late rice seasons, but the addition of Zn fertilizer (90 kg/hm2) to soil and through foliar spraying (0.2 g·L-1 and 0.4 g·L-1) had no significant effects on the pH or CEC of the soil. ② LS application decreased concentrations of TCLP-Cd and CaCl2-Cd in the soils, by 11.5%-38.8% and 24.0%-81.0%, respectively, while neither of the treatments involving the addition of Zn fertilizer to soil or through foliar spraying had any significant effects on the concentrations of TCLP-Cd and CaCl2-Cd. ③Single treatments involving only LS amendment, Zn fertilizer in soil, or foliar spraying of Zn fertilizer also reduced Cd concentrations in brown rice, but to a lesser degree than the combined treatments. The combined treatments (L1Z1F1, L1Z1F2, L2Z1F1, and L2Z1F2) reduced Cd concentrations in brown rice by 64.9%-67.5% and 56.1%-80.6%, for early and late rice, respectively, while L2Z1F1 (4500 kg·hm-2 LS+90 kg·hm-2 Zn fertilizer+foliar spraying 0.2 g·L-1 Zn fertilizer) resulted in the largest reduction in Cd concentration in brown rice. ④ The Cd/Zn ratio in brown rice was significantly positively correlated with Cd concentrations, indicating that increased Zn concentration in different rice tissues was one of the key reasons for decreased Cd concentration in brown rice. Clearly, as a remediation technology, combining LS amendments with zinc fertilizer is an effective method for achieving the safe utilization of moderately and severely Cd contaminated paddy fields, by effectively inhibiting the uptake, accumulation, and transportation of Cd in rice plants and decreasing Cd concentrations in brown rice.

Nano-Fe3O4-modified biochar promotes the formation of iron plaque and cadmium immobilization in rice root.

Rice as a paddy field crops, iron-containing materials application could induce its iron plaque formation, thereby affecting cadmium (Cd) transportation in the rhizosphere and its uptake in root. In this study, a hydroponic experiment was conducted to investigate the effects of three exogenous iron materials, namely nano-Fe3O4-modified biochar (BC-Fe), chelated iron (EDTA-Fe), and ferrous sulfate (FeSO4), on the iron plaque formation on the surface of rice root, and to investigate the effects of formed iron plaque on the absorption, migration, and transportation of Cd and Fe in rice plant. The results showed that yellow-brown and brown iron plaque was formed on surface cells of the Fe-treated rice root, and some black particles were embedded in the iron plaque formed by BC-Fe. The proportion of crystallized iron plaque (31.8%-35.9%) formed by BC-Fe was much higher than that formed by EDTA-Fe and FeSO4. The Cd concentrations in the crystallized iron plaque formed by BC-Fe were 7.64-13.0 mg·kg-1, and increased with the increasing of Fe concentrations in the plaque. The Cd translocation factor from root to stem (TFr-s) and the Cd translocation factor from stem to leaf (TFs-l) with BC-Fe treatment decreased by 84.7% and 80.0%, respectively. The results demonstrated that application BC-Fe promoted the formation of iron plaque and enhanced the sequestration of Cd and Fe in roots, thus reduced the transportation and accumulation of Cd in aerial rice tissues.

The Fe3O4-modified biochar reduces arsenic availability in soil and arsenic accumulation in indica rice (Oryza sativa L.).

Arsenic (As)-contaminated paddy soil could result in elevated levels of As in rice plants and sequentially harm human health. The Fe3O4-modified biochar (NBC-Fe) prepared by the coprecipitation method was applied in a pot experiment to investigate its effect on mobility and bioavailability of As in soil and to reduce As accumulation in rice tissues (brown rice, husks, spikelets, leaves, stems, and roots). Compared with non-application (CK), application of NBC-Fe significantly increased the cation exchange capacity (CEC), decreased As availability, and raised the As concentration of crystalline hydrous oxide-bound fraction in the soil. The addition of 0.05-1.6% (w/w) NBC-Fe significantly reduced the As concentrations in brown rice by 9.4-47.3%, which was lower than the level set by the National Food Safety Standards of China (0.2 mg/kg). The NBC-Fe treatment decreased As concentrations in iron plaque (DCB-As), and the DCB-As had the very significant correlations (P < 0.01) with the As concentrations in different rice tissues (brown rice, husks, spikelets, leaves, stems, and roots). The NBC-Fe immobilized As to decrease As availability in soil and increased the amount and thickness of iron plaque to sequester As on the surfaces of rice root. This study demonstrates that NBC-Fe is a promising soil amendment for the remediation of As-contaminated soil, therefore reducing As accumulation in rice plant and safety risks for rice consumption.

[Regulation Control of a Tribasic Amendment on the Chemical Fractions of Cd and As in Paddy Soil and Their Accumulation in Rice].

A pot experiment was conducted to identify the effect of a tribasic amendment (limestone+diatomite+ferric sulfate, LDF) on chemical fractions of Cd and As in paddy soils and their accumulation in brown rice. LDF was set to seven levels (0, 0.5, 1.0, 2.0, 4.0, 8.0, and 16.0 g·kg-1) based on the quality ratio, and two genotypes of rice were planted (Huanghuazhan and T-you 272). The results show that:① The application of LDF increased the rhizosphere soil pH of two varieties of rice, Huanghuazhan and T-you 272, by 0.01-0.42 and 0.11-0.54, respectively, and decreased the concentrations of EX-Cd by 11.1%-61.1% and 26.5%-52.9%, respectively, and the concentrations of EX-As by 8.2%-60.0% and 5.6%-49.9%, respectively. ② Application of LDF promoted the transformation of soil Cd and As from soluble to insoluble forms. Although the trends of the rhizosphere soils of the two rice varieties were not consistent, the application of LDF could decrease the proportion of EX-Cd and increase the proportion of Fe/Mn-Cd, Org-Cd, and O-Cd, which was accompanied by the reduction of the proportion of EX-As and an increase in the proportion of Ca-As. ③ The concentrations of Cd, As, and Fe in the iron plaque decreased by applying LDF, while the concentration of Mn increased, and the maximum increase of Mn could reach 124.2%. ④ Application of LDF decreased the concentrations of Cd in brown rice of the two varieties of rice by 64.6% and 65.9%, respectively, and decreased that of As by 37.0% and 42.5%, respectively. The effect on the concentrations of inorganic As was not significant. When the application amount of LDF was 2-16 g·kg-1, the concentrations of Cd and inorganic As in T-you 272 brown rice were both under 0.2 mg·kg-1, and when the application amount was 16 g·kg-1, the concentrations of Cd and inorganic As in Huanghuazhan brown rice were both under 0.2 mg·kg-1. In actual agricultural production, the application amount of LDF can be adjusted according to the soil pollution levels and the rice varieties.

[Key Growth Stage of Pb Accumulation in Rice Through a Hydroponic Experiment with Pb Stress].

Pb accumulation in rice varies significantly at different growth stages. In this study, a hydroponic experiment was conducted to study the effects of exogenous Pb stress on Pb accumulation and transportation in rice plants and determine the key rice growth stages of Pb accumulation and their contribution to the Pb content in brown rice. For the hydroponic experiment, 0.5 mg ·L-1 of exogenous Pb was added to rice at different growth stages, including the tillering stage, jointing stage, booting stage, filling stage, dough stage, maturing stage, and whole growth stage (103 d) of the rice plant. Another treatment of the whole growth stage without Pb stress was established as the control. The results showed that: ① There were no significant effects of Pb stress in any single stage on the plant height, tiller number, and biomass, but the rice plant height and biomass significantly decreased under the Pb stress for the whole growth period. ② Pb contents in different tissues at the maturing stage of the rice plant were in the order of root > stem node 1 > other stem nodes > root rhizome > sti > leaf > ear > husk > brown rice. Pb contents in brown rice ranged from 0.1 to 1.2 mg ·kg-1 for all exogenous Pb treatments, and ranked in the growth stages as booting stage > jointing stage > tillering stage > filling stage > maturing stage > dough stage. ③ The relative contribution rates of Pb accumulation in the whole rice plants were relatively high during the reproductive growth period (filling stage, dough stage, and maturing stage), while those in the above-ground parts of the rice plants were relatively high during the vegetative growth stage (tillering stage, jointing stage, booting stage). ④ The booting stage was the key growth period for Pb accumulation in brown rice. Pb stress at this stage contributed 43.3% of Pb content in brown rice, followed by Pb stress at the jointing stage and the tillering stage, with contribution rates of 24.4% and 21.3%, respectively. ⑤ Water management regimes, application of amendments, or leaf resistance control techniques should be applied appropriately at the booting stage of the rice plants to reduce Pb accumulation in brown rice and to realize the safe use of rice paddies polluted with Pb.

[Effects of Different Treatments with Water Management Combined with Leaf Spraying Silicon Fertilizer on Cd Accumulation in Rice].

A field experiment involving eight treatments with water management combined with leaf spraying silicon fertilizer was conducted in a paddy field heavily contaminated with Cd (2.83 mg·kg-1) to study the effects of these treatments on rice growth and Cd accumulation in different rice tissues. The results showed that:① the treatments had no significant effects on rice plant height or number of tillers, but increased the biomass of brown rice by 1.7% to 25.0%. Among the eight treatments, that of water flooding during the rice maturation period plus leaf spraying silicon fertilizer (CY) resulted in the highest amount of brown rice yield. ② The treatment of conventional water management plus leaf spraying silicon fertilizer (Si) had no significant effect on the exchangeable Cd content and TCLP extractable Cd content in soil, whereas the other treatments reduced the exchangeable Cd content by 7.8%-42.6% and the TCLP extractable Cd content by 20.0%-40.8%. ③ The Si treatment could reduce the Cd content in various rice tissues, with an overall decrease of 19.0% in brown rice. The other treatments significantly reduced the Cd content in various rice tissues. The treatment of moisture during the rice maturation period plus leaf spraying silicon fertilizer (CS) resulted in the highest reduction in the Cd content in brown rice (44.0%), and was followed by the treatments of batch-type water flooding during the entire rice growth period plus leaf spraying silicon fertilizer (JX; 36.4%), and moisture during the rice pustulation period plus leaf spraying silicon fertilizer (GS; 31.8%). ④ For paddy-fields that are contaminated with Cd to medium and heavy levels, the CS and JX treatments are recommended to manage rice production in order to significantly reduce the Cd content of brown rice whilst having little effect on the rice yield.

[Key Stage of As Accumulation in Rice Under As Stress at Different Growth Stages].

The absorption and accumulation of As at different stages of rice growth are significantly different. To study the key growth stages of As accumulation in brown rice and to determine the contribution of As accumulation at different growth stages to As contribution in brown rice, a rice hydroponics experiment was carried out by adding external As during the different rice growth stages: tillering stage (30 d), jointing stage (16 d), booting stage (13 d), filling stage (17 d), dough stage (15 d), maturity stage (13 d), and full growth period (104 d). The results showed that: ① As stress at different growth stages had a significant effect on the biomass of rice plants. In comparison with the control CK, the treatments of As stress with five single-stages decreased the rice biomass, except during the tillering stage. Among these treatments, although the As stress at booting stage had the lowest rice biomass, the biomass of all the rice plants were higher than that for As stress in the full growth period. ② The content of As in brown rice for all the treatments of As stress at six single-stages, ranging from 0.08-0.24 mg ·kg-1, was higher than that of CK. Among these six treatments, the As stress at booting stage had the highest As content in brown rice, accounting for 64.9% of the As content in the full growth period. ③ The accumulation of As in brown rice for all the treatments of As stress at six single-stages was higher than that of CK, and ranged from 1.4-1.5 µg ·plant-1. Among these six treatments, the As stress at booting stage had the highest As accumulation, followed by filling stage. The accumulation of As in brown rice treated with As stress was highest in the full growth period with a value of 5.7 µg ·plant-1. ④ The relative contribution of As stress at the booting stage was the highest for the As accumulation in brown rice and reached to 40.3%, followed by filling stage with 26.0%. Therefore, the rice growth stages of the booting stage and filling stage were the key stages of As accumulation in brown rice.

[Effects of Exogenous Phosphorus on Rice Growth and Cadmium Accumulation and Transportation Under Cadmium Stress].

In this study, we investigated the effects of exogenous phosphorus on the accumulation and transportation of cadmium in rice plants through a hydroponic experiment. In the experiment, the rice variety was Huanghuazhan, P solution concentrations were 10.0-45.0 mg ·L-1 that was made using NaH2PO4, and Cd solution concentrations were 0.1 mg ·L-1 and 0.2 mg ·L-1. The results showed that: ① the biomass in all parts of rice plants and contents of photosynthetic pigment (chlorophyll a, chlorophyll b, and carotenoid) increased gradually with an increase in exogenous P. ② Content of Cd in rice stems, leaves, husk, and brown rice increased gradually with an increase in the amounts of exogenous P. The content of Cd in brown rice increased by 2.8%-22.8% and 40.9%-61.8% when treated with Cd concentrations of 0.1 mg ·L-1 and 0.2 mg ·L-1 in hydroponic solutions, respectively. ③ Cd accumulation in rice plants was accelerated due to the application of exogenous P. Cd accumulating amounts increased from 395.1 µg ·plant-1 to 542.6 µg ·plant-1 and 639.6 µg ·plant-1 to 1082.0 µg ·plant-1 when treated with Cd concentrations of 0.1 and 0.2 mg ·L-1 in hydroponic solutions, respectively. ④ With an increase in the applied amounts of exogenous P, the P/Cd quality ratio in rice roots increased, while those in rice stems, leaves, husks, and brown rice decreased; meanwhile, the Cd transfer coefficients from root to stem (TFroot-stem) and stem to leaf (TFstem-leaf) increased. This showed that there was a certain synergistic effect between P and Cd in the rice parts. Finally, the application of exogenous P promoted the transfer of Cd from the rice root to other rice tissues, resulting in a synergistic effect on Cd accumulation and transportation in various rice tissues and increased Cd contents in brown rice.