Exploration of Cd transformations in Cd spiked and EDTA-chelated soil for phytoextraction by Brassica species.

The study of soil cadmium (Cd) fractionation has become the need of the hour due to phytoextraction of Cd heavy metal by indigenous Brassica species of northwest India. The present study was conducted to explore the Cd speciation in soils treated with Cd (0, 5.0, 10.0, 20.0, 40.0, and 80.0 mg kg-1 soil) and synthetic chelate ethylene diamine tetraacetic acid (EDTA-0, 1.0 and 2.0 g kg-1 soil) planted under three Brassica species (Brassica juncea L., Brassica campestris L., and Brassica napus L). The studied Cd fractions viz. exchangeable and water-soluble (EX + WS), carbonate (CARB), organic matter (OM), Mn oxide (MnOX), amorphous Fe oxide (AFeOX), crystalline Fe oxide (CFeOX), and residual (RES) differed in their Cd content in soils under three investigated Brassica species. Among all plantations, B. juncea reduced the highest soil Cd content of EX + WS form which reflected its bioavailability. The Cd supplementation significantly enhanced the Cd concentration in all Cd forms with EX + WS Cd form exhibiting higher increase even at low Cd level (5.0 mg kg-1), whereas the EDTA addition did not influence Cd fractions. The application of EDTA @ 1.0 g kg-1 soil proved beneficial as it enhanced the metal mobility for plant extraction. All species positively significantly correlated (r = 0.648** to 0.747**) with all Cd fractions but except B. juncea all confronted reduction in their total biomass. In nutshell, it suggested that Brassica species having large plant biomass could be considered as a potential candidate for phytoremediation.

Effects of Pseudomonas TCd-1 on rice (Oryza sativa) cadmium uptake, rhizosphere soils enzyme activities and cadmium bioavailability under cadmium contamination.

Microbial remediation is a promising technique to reduce Cd accumulation in rice (Oryza sativa). In present study, a set of pot experiments were conducted to evaluate the effects of Cd-tolerate Pseudomonas TCd-1 inoculation on rice Cd uptake, soil enzyme activities and Cd bioavailability in the rhizosphere soils under Cd contaminated conditions. The results showed that at the ripening stage, with the inoculation of TCd-1, Cd contents in root, culm, leaf, hull and brown rice significantly reduced by 60.7%, 47.7%, 50.6%, 58.1% and 47.9%, respectively, and the cadmium bioconcentration factor (BCF) of rice lowered by 66.2% under 5 mg kg-1 Cd treatment. At the meantime, in the rhizosphere soils, pH increased by 0.05, the contents of exchangeable Cd (EX-Cd) and Fe-Mn oxides (OX-Cd) increased by 107.8% and 33.5%, whereas organic matter (OM-Cd) and residual (Res-Cd) decreased by 31.9% and 60.0%, respectively. The activity of acid phosphatase (ACP) increased by 28.3%, catalase (CAT), saccharase (SUC) activity decreased by 28.5% and 26.0%. Similarly, the Cd contents in root, culm, leaf, hull and brown rice reduced by 42.1%, 42.5%, 58.0%, 50.3%, and 68.8%, respectively, and the BCF lowered by 57.1%, under 10 mg kg-1 Cd treatment. Simultaneously, the soil pH increased by 0.06, the activities of CAT, SUC, urease (URE), ACP decreased by 26.4%, 34.6%, 63.8% and 15.3%, respectively. Furthermore, the correlation analysis showed that the inoculation of TCd-1 changed the correlation between rice Cd content and the biomass of roots, leaves, soil pH, CAT, PPO, URE activities, OM-Cd in rhizosphere soils. It suggested that Pseudomonas TCd-1 effectively reduced Cd uptake and Cd accumulation in rice was closely linked to the changes of soil pH, enzyme activities and Cd availability.

The Effects of Cadmium Exposure on Cadmium Fractionation and Enzyme Activities in the Rhizosphere of Two Radish Cultivars (Raphanus sativus L.).

The effects of increasing Cd additions on plant growth and Cd fractionation and enzyme activities in rhizosphere soil of two radish cultivars were investigated. The results showed that Cd concentrations in shoot and root of cultivar 4 were both higher than for cultivar 19 under different Cd levels. Compared with cultivar 19, the total, shoot and root biomasses of cultivar 4 were significantly reduced with increasing Cd levels. A decrease in soil pH was observed for cultivar 4. The exchangeable Cd concentration of soil from cultivar 4 was significantly higher than for soil from cultivar 19, while the carbonate-bound Cd concentration of soil from cultivar 4 was significantly lower than for cultivar 19. Enzyme activities, especially acid phosphatase activity, were more susceptible to Cd in soil from cultivar 4. These results indicated that cultivar 19 exhibits a stronger ability to adapt to Cd stress than cultivar 4.

The Cd immobilization mechanisms in paddy soil through ureolysis-based microbial induced carbonate precipitation: Emphasis on the coexisting cations and metatranscriptome analysis.

Microbial induced carbonate precipitation (MICP) can immobilize metals and reduce their bioavailability. However, little is known about the immobilization mechanism of Cd in the presence of soil cations and the triggered gene expression and metabolic pathways in paddy soil. Thus, microcosmic experiments were conducted to study the fractionation transformation of Cd and metatranscriptome analysis. Results showed that bioavailable Cd decreased from 0.62 to 0.29 mg/kg after 330 d due to the MICP immobilization. This was ascribed to the increase in carbonate bound, Fe-Mn oxides bound, and residual Cd. The underlying immobilization mechanisms could be attributed to the formation of insoluble Cd-containing precipitates, the complexation and lattice substitution with carbonate and Fe, Mn and Al (hydr)oxides, and the adsorption on functional group on extracellular polymers of cell. During the MICP immobilization process, up-regulated differential expression urease genes were significantly enriched in the paddy soil, corresponding to the arginine biosynthesis, purine metabolism and atrazine degradation. The metabolic pathway of bacterial chemotaxis, flagellum assembly, and peptidoglycan biosynthesis and the expression of cadA gene related to Cd excretion enhanced Cd resistance of soil microbiome. Therefore, this study provided new insights into the immobilization mechanisms of Cd in paddy soils through ureolysis-based MICP process.

The Aging Process of Cadmium in Paddy Soils under Intermittent Irrigation with Acid Water: A Short-Term Simulation Experiment.

Cadmium (Cd)-contaminated paddy soils are a big concern. However, the effect of irrigation with acid water on the migration and transformation of Cd and the effect of alternating redox conditions caused by intermittent irrigation on Cd aging processes in different depths of paddy soils are unclear. This study revealed Cd fractionation and aging in a Cd-contaminated paddy soil under four irrigation periods with acid water and four drainage periods, by applying a soil columns experiment and a sequential extraction procedure. The results showed that the dynamic changes of soil pH, oxidation reduction potential (ORP), iron (Fe) oxides and dissolved organic carbon (DOC) throughout the intermittent irrigation affected the transformation of Cd fractions. After 32 days, the proportion of exchangeable Cd (F1) to the total Cd decreased with a reduction of 24.4% and 20.1% at the topsoil and the subsoil, respectively. The labile fractions of Cd decreased, and the more immobilizable fractions of Cd increased in the different depths of soils due to the aging process. Additionally, the redistribution of the Fe and Mn oxide-bound Cd (F3) and organic matter and secondary-sulfide-bound Cd (F4) occurred at different depths of soils during the incubation time. Overall, the bioaccessibility of Cd in the subsoil was higher than that in the topsoil, which was likely due to the leaching and accumulation of soluble Cd in the deep soil. In addition, the aging processes in different depths of soils were divided into three stages, which can be mainly described as the transformation of F1 into F3 and F4.

Variations in cadmium accumulation among amon rice cultivars in Bangladesh and associated human health risks.

Rice consumption is one of the major cadmium (Cd) exposure routes for human. Bangladeshi people have historically subsisted on a rice-based diet; however, only a few reports have investigated Cd accumulation by different rice cultivars in Bangladesh. This study was designed to investigate the uptake and accumulation of Cd in different rice cultivars and associated health risks to humans eating rice. A pot experiment was conducted to grow eight amon rice varieties under control, 5 and 10 mg Cd/kg soil under open air conditions. After harvesting the Cd fractionation, bioavailable Cd and rice grain Cd content were determined. Cd spiked as Cd2+ enriched the Cd bioavailability to plant by 35% (in 5 mg/kg stress) and 85% (in 10 mg/kg stress). There were variations among the rice varieties in their ability to accumulate Cd in grain and this was found to be 15-fold higher under control conditions. Grain Cd content significantly differed among the rice varieties at each level of soil Cd. In this study, BR-52 emerged as the most Cd-safe cultivar followed by BR-75, Rani salut, BR-71, BR-49, BR-76, BR-87 and lastly, BINA-7. Most of the agronomic parameters of rice concerning yield were affected by both rice varieties and soil Cd level. In different rice varieties, rhizosphere pH increased through root exudation which ultimately produced equilibria among the five major soil Cd fractions so that Cd became bioavailable to plants. All rice varieties showed high hazard quotient (HQ) values under Cd stress conditions and posed a risk to human health. For noncarcinogenic health risk assessment through HQ, we recommend 0.1 mg Cd/kg rice grain be used as the maximum allowable concentration (MAC) in calculating health risk for Bangladeshi people.

Earthworms, Rice Straw, and Plant Interactions Change the Organic Connections in Soil and Promote the Decontamination of Cadmium in Soil.

The joint effects of earthworms and crop straw on toxic metal speciation are not clear, and very limited information is available regarding the effects of their interaction on Cd mobility in Cd contaminated soil or in remediation processes involving plants. This study evaluated their impacts on Cd mobile form changes in soil and their effects on Cd uptake by plants. Treatments included both planted and unplanted-Cd-contaminated soil with or without rice straw and/or earthworms. The results revealed that earthworms, rice straw, and plant interactions change the Cd mobile forms in soil. The order of Cd concentration of different chemical forms was as follows: exchangeable > residual > bound to Fe-Mn oxide > bound to organic matter for earthworms, and exchangeable > bound to organic matter > residual > bound to Fe-Mn oxide for rice straw treatment, with a recovery rate of 96 ± 3%. The accumulation of Cd in plants increased in the presence of earthworms and decreased in the presence of rice straw. FT-IR spectra indicated that the degradation of rice straw increases C⁻O, C⁻O⁻H, C⁻H, and O⁻H functional groups which could complex with Cd ions. These findings highlighted that earthworms' activities and crop straw can modify soil properties and structure and promote the remediation of heavy metal. This study suggests that the ecological context of remediation instead of being limiting on soil-earthworms-plant interaction, should integrate the natural resources forsaken which can provide a positive influence on both plant health and the remediation of heavy metal in contaminated soil.