Regulation of rhizosphere microenvironment by rice husk ash for reducing the accumulation of cadmium and arsenic in rice

Rich-silicon rice husk ash increases iron plaque formation and decreases cadmium and arsenic accumulation in rice seedlings

Combined Cd (cadmium) and As (arsenic) pollution in cultivated land affects the safety of crops production and endangers human health. Rice (Oryza sativa L.) is a crop that uptakes Si (silicon), and Si can effectively promote rice growth and mitigate heavy metal toxicity. This study examined the effect and mechanism of Si-rich amendment (HA) prepared by aerobic combustion of rice husk on Cd and As accumulation in iron plaque and rice seedlings via hydroponic experiments. HA enhanced the vitality of rice growth because of its Si content and increased the amount of amorphous fraction iron plaques, furthermore, Cd content was decreased while the As was increased in both amorphous fraction and crystalline fraction iron plaques, resulting in the contents of Cd and As decreases by 10.0%-38.3% and 9.6%-42.8% for the shoots, and by 13.4%-45.2% and 9.9%-20.0% for the roots, respectively. In addition, X-ray diffraction and X-ray photoelectron spectroscopy illustrated significantly more Fe2O, MnO2 and MnO in the iron plaque after HA supply and the simultaneous existence of Mn-As and Mn-Si compounds. This result revealed less Cd from iron plaque and more As retention with HA supply, reducing the amount of Cd and As up taking and accumulation by rice seedlings. HA is beneficial to rice growth and reduce the absorption of heavy metals in plants. At the same time, HA is environmentally friendly, it can be used for the remediation of paddy fields contaminated by Cd and As.

Simultaneous immobilization strategy of anionic metalloids and cationic metals in agricultural systems: A review

Concurrent heavy metals remediation in natural environments poses significant challenges due to factors like metal speciation and interactions with soil moisture. This review focuses on strategies for immobilizing both anionic and cationic metals simultaneously in soil-crop systems. Key approaches include water management, biochar utilization, stabilizing agents, nanotechnology, fertilization, and bioremediation. Sprinkler or intermittent irrigation combined with soil amendments/biochar effectively immobilizes As/Cd/Pb simultaneously. This immobilization occurs through continuous adsorption-desorption, oxidation-reduction, and precipitation mechanisms influenced by soil pH, redox reactions, and Fe-oxides. Biochar from sources like wine lees, sewage sludge, spent coffee, and Fe-nanoparticles can immobilize As/Cd/Pb/Cr/Co/Cu/Zn together via precipitation. In addition, biochar from rice, wheat, corn straw, rice husk, sawdust, and wood chips, modified with chemicals or nanoparticles, simultaneously immobilizes As and Cd, containing higher Fe3O4, Fe-oxide, and OH groups. Ligand exchange immobilizes As, while ion exchange immobilizes Cd. Furthermore, combining biochar especially with iron, hydroxyapatite, magnetite, goethite, silicon, graphene, alginate, compost, and microbes-can achieve simultaneous immobilization. Other effective amendments are selenium fertilizer, Ge-nanocomposites, Fe-Si materials, ash, hormone, and sterilization. Notably, combining nano-biochar with microbes and/or fertilizers with Fe-containing higher adsorption sites, metal-binding cores, and maintaining a neutral pH could stimulate simultaneous immobilization. The amendments have a positive impact on soil physio-chemical improvement and crop development. Crops enhance production of growth metabolites, hormones, and xylem tissue thickening, forming a protective barrier by root Fe-plaque containing higher Fe-oxide, restricting upward metal movement. Therefore, a holistic immobilization mechanism reduces plant oxidative damage, improves soil and crop quality, and reduces food contamination.

Rhizosphere effect on the relationship between dissolved organic matter and functional genes in contaminated soil

Arsenic contamination in a mining area is a potential threat to the local population. In the context of one-health, biological pollution in contaminated soil should be known and understandable. This study was conducted to clarify the effects of amendments on arsenic species and potential threat factors (e.g., arsenic-related genes (AMGs), antibiotic resistance genes (ARGs) and heavy-metal resistance genes (MRGs)). Ten groups (control (CK), T1, T2, T3, T4, T5, T6, T7, T8, and T9) were set up by adding different ratio of organic fertilizer, biochar, hydroxyapatite and plant ash. The maize crop was grown in each treatment. Compared with CK, the bioavailability of arsenic was reduced by 16.2%-71.8% in the rhizosphere soil treatments, and 22.4%-69.2% in the bulk soil treatments, except for T8. The component 2 (C2), component 3 (C3) and component 5 (C5) of dissolved organic matter (DOM) increased by 22.6%-72.6%, 16.8%-38.1%, 18.4%-37.1%, respectively, relative to CK in rhizosphere soil. A total of 17 AMGs, 713 AGRs and 492 MRGs were detected in remediated soil. The humidification of DOM might directly correlate with MRGs in both soils, while it was influenced directly on ARGs in bulk soil. This may be caused by the rhizosphere effect, which affects the interaction between microbial functional genes and DOM. These findings provide a theoretical basis for regulating soil ecosystem function from the perspective of arsenic contaminated soil.