Effect of Cd toxicity on root morphology, ultrastructure, Cd uptake and accumulation of wheat under intercropping with Solanum nigrum L.

In order to evaluate the effect of intercropping of the hyperaccumulator, Solanum nigrum L. and wheat on the absorption and accumulation of cadmium (Cd) in wheat. The experiment was conducted with three replicates, which conducted on four Cd concentrations (0, 20, 40, 60 µmol L-1) in the Hoagland solution and using two planting patterns [monoculture wheat (MW), intercropping wheat and Solanum nigrum L. (IW，IS)]. The results showed that the addition of Cd in the solutions reduced the total root length by 19.08-55.98%, total root area by 12.35-44.48%, and total root volume by 16.01-46.00% of wheat plants. Intercropping with Solanum nigrum L. significantly reduced Cd contents by 28.3-47.2% and Cd accumulations by 10.08-32.43% in the roots of wheat. Transmission electron microscope (TEM) revealed that the root-tip cells of the monoculture wheat treated with Cd exhibited swollen spheres of intracellular mitochondria, disorderly arranged inner ridges of mitochondria, some damaged mitochondrial membranes, and deformed nuclear membranes. Many dense electron particles in the form of Cd were deposited in the cell gap, and the cell nucleus became smaller or even disappeared. Under the same Cd concentrations, root-tip cells of intercropped wheat showed much less density of electron particles, starch granules, and the damage to the nucleus and nuclear membrane by Cd.These results indicated that intercropping with Solanum nigrum L. reduced the Cd toxicity to wheat roots and decreased Cd uptake and accumulation in both the shoots and roots of the wheat.

Response of methylmercury in paddy soil and paddy rice to pristine biochar: A meta-analysis and environmental implications.

Biochar has received increased research attention due to its effectiveness in mitigating the potential risks of mercury (Hg) in agricultural soils. However, there is a lack of consensus on the effect of pristine biochar on the net production, availability, and accumulation of methylmercury (MeHg) in the paddy rice-soil system. As such, a meta-analysis with 189 observations was performed to quantitatively assess the effects of biochar on Hg methylation, MeHg availability in paddy soil, and the accumulation of MeHg in paddy rice. Results suggested that biochar application could significantly increase the production of MeHg in paddy soil by 19.01%; biochar could also decrease the dissolved and available MeHg in paddy soil by 88.64% and 75.69%, respectively. More importantly, biochar application significantly inhibited the MeHg accumulation in paddy rice by 61.10%. These results highlight that biochar could decrease the availability of MeHg in paddy soil and thus inhibit MeHg accumulation in paddy rice, although it might facilitate the net production of MeHg in paddy soil. Additionally, results also indicated that the biochar feedstock and its elementary composition significantly impacted the net MeHg production in paddy soil. Generally, biochar with a low carbon content, high sulfur content, and low application rate might be beneficial for inhibiting Hg methylation in paddy soil, meaning that Hg methylation depends on biochar feedstock. These findings suggested that biochar has great potential to inhibit MeHg accumulation in paddy rice, and further research should focus on selecting biochar feedstock to control Hg methylation potential and determine its long-term effects.

A new insight into the role of iron plaque in arsenic and cadmium accumulation in rice (Oryza sativa L.) roots.

Iron plaque, naturally iron-manganese (hydr)oxides adhered to the surface of rice roots, controls the sequestration and accumulation of arsenic (As) and cadmium (Cd) in the paddy soil-rice system. However, the effects of the paddy rice growth on the iron plaque formation and As and Cd accumulation of rice roots are often neglected. This study explores the distribution characteristics of iron plaques on rice roots and their effects on As and Cd sequestration and uptake via cutting the rice roots into 5 cm segments. Results indicated that the percentages of rice root biomass of 0-5 cm, 5-10 cm, 10-15 cm, 15-20 cm, and 20-25 cm are 57.5 %, 25.2 %, 9.3 %, 4.9 %, and 3.1 %, respectively. Iron (Fe) and manganese (Mn) concentrations in iron plaques on rice roots of various segments are 41.19-81.11 g kg-1 and 0.94-3.20 g kg-1, respectively. Increased tendency of Fe and Mn concentrations from the proximal rice roots to the distal rice roots show that iron plaque is more likely to deposit on the distal rice roots than proximal rice roots. The DCB-extractable As and Cd concentrations of rice roots with various segments are 694.63-1517.23 mg kg-1 and 9.00-37.58 mg kg-1, displaying a similar trend to the distribution characteristics of Fe and Mn. Furthermore, the average transfer factor (TF) of As (0.68 ± 0.26) from iron plaque to rice roots was significantly lower than that of Cd (1.57 ± 0.19) (P < 0.05). There was a significant positive correlation between the Cd sequestration in iron plaque and the Cd accumulation in rice roots (R = 0.97, P < 0.01). Still, a similar correlation wasn't observed between As sequestration in iron plaque and As accumulation in rice roots (R = -0.04, and P > 0.05). These results indicated that the formed iron plaque might act as a barrier to As uptake by rice roots and a facilitator to Cd uptake. This study provides insight into the role of iron plaque in the sequestration and uptake of As and Cd in paddy soil-rice systems.

Identification of heavy metal pollutants and their sources in farmland: an integrated approach of risk assessment and X-ray fluorescence spectrometry.

Investigation and assessment of farmland pollution require an efficient method to identify heavy metal (HM) pollutants and their sources. In this study, heavy metals (HMs) in farmland were determined efficiently using high-precision X-ray fluorescence (HDXRF) spectrometer. The potential ecological risk and health risk of HMs in farmland near eight villages of Wushan County in China were quantified using an integrated method of concentration-oriented risk assessment (CORA) and source-oriented risk assessment (SORA). The CORA results showed that Cd in farmland near the villages of Liuping (LP) and Jianping (JP) posed a "very high" potential ecological risk, which is mainly ascribed to soil Cd (single potential ecological risk index ([Formula: see text]) of Cd in villages LP and JP, [Formula: see text] = 2307 and 568 > 320). A "moderate" potential ecological risk was present in other six villages. The overall non-carcinogenic risk (hazard index (HI) = 1.2 > 1) of HMs for children in village LP was unacceptable. The contributions of HMs decrease in the order of Cr > As > Cd > Pb > Ni > Cu > Zn. The total carcinogenic risk (TCR = 2.1 × 10-4 > 1.0 × 10-4) of HMs in village LP was unacceptable, with HMs contributions decreasing in the order of Cr > Ni > Cd > As > Pb. Furthermore, three source profiles were assigned by the positive matrix factorization: F1: agricultural activity; F2: geological anomaly originating from HMs-rich rocks; F3: the natural geological background. According to the results of SORA, F2 was the highest contributor to PER in village LP, up to 64.4%. Meanwhile, the contributions of three factors to HI in village LP were 19.0% (F1), 53.6% (F2), and 27.4% (F3), respectively. It is worth noting that TCR (1.2 × 10-4) from F2 surpassed the threshold of 1.0 × 10-4, with an unacceptable carcinogenic risk level. As mentioned above, the HM pollutants (i.e., Cd and Cr) and their main sources (i.e., F2) in this area should be considered. These results show that an integrated approach combining risk assessments with the determination of HM concentration and identification of HM source is effective in identifying HM pollutants and sources and provides a good methodological reference for effective prevention and control of HM pollution in farmland.

Simultaneous adsorption of As(III) and Cd(II) by ferrihydrite-modified biochar in aqueous solution and their mutual effects.

A simply synthetic ferrihydrite-modified biochar (Fh@BC) was applied to simultaneously remove As(III) and Cd(II) from the aqueous solution, and then to explore the mutual effects between As(III) and Cd(II) and the corresponding mechanisms. The Langmuir maximum adsorption capacities of As(III) and Cd(II) in the single adsorbate solution were 18.38 and 18.18 mg g-1, respectively. It demonstrated that Fh@BC was a potential absorbent material for simultaneous removal of As(III) and Cd(II) in aqueous solution. According to the XRF, SEM-EDS, FTIR, XRD, and XPS analysis, the mechanisms of simultaneous removal of As(III) and Cd(II) by Fh@BC could be attributable to the cation exchange, complexation with R-OH and Fe-OH, and oxidation. Moreover, the mutual effect experiment indicated that Cd(II) and As(III) adsorption on Fh@BC in the binary solution exhibited competition, facilitation and synergy, depending on their ratios and added sequences. The mechanisms of facilitation and synergy between Cd(II) and As(III) might include the electrostatic interaction and the formation of both type A or type B ternary surface complexes on the Fh@BC.

Risk Identification of Heavy Metals in Agricultural Soils from a Typically High Cd Geological Background Area in Upper Reaches of the Yangtze River.

This study adopted two risk assessment models to estimate the potential risk of heavy metals (HMs) in agricultural soils from a high Cd geological background area. Results were as follows: HMs posed an extremely high potential ecological risk (PER) (PER = 2051 > 1200) ascribed to Cd contribution. The overall non-carcinogenic risk (HI) of HMs for children (HI > 1) was unacceptable. Contributions of HMs to HI generally decreased in the order of Cr > As > Pb > Cd > Ni > Cu > Zn. The total carcinogenic risk (TCR) value for adults (2.1 × 10-4) and children (2.2 × 10-4) surpassed the reference value (1.0 × 10-4), indicating that the risks were unacceptable. Contributions of HMs to TCR generally decreased in the order of Cr ≈ Ni > Cd > As > Pb. These results suggested that risk control policies should not only consider the potential ecological risk of Cd but also take into consideration of the carcinogenic and non-carcinogenic risk of Cr and Ni in a typically high Cd geological background area.

Spatial Distribution of As and Cd in Co-contaminated Soils Within the Rice Root Microzone.

Soil samples were collected from the As and Cd co-contaminated demonstration area to analyze the spatial distributions of As and Cd in soils within the rice root microzone using the treatment of soil cubes (50 mm × 50 mm × 50 mm). The results indicated that there was no significant difference in the spatial distributions of total As or Cd in the microzone with the percentage of root weight, horizontal distances (HD), and vertical depths (VD). Interestingly, available As or Cd increased with the increasing HD, and both of them showed a significant difference (p < 0.05) between < 75 mm and ≥ 120 mm. The availability of As or Cd increased from the center to the edge of the rice root microzone. Moreover, the risk assessment code (RAC) showed that the site with a high potential risk of As or Cd was located on the edge of the rice root microzone. This phenomenon implied that cultivating paddy rice in the low potential risk microzone might have a low accumulative risk.

Does biochar inhibit the bioavailability and bioaccumulation of As and Cd in co-contaminated soils? A meta-analysis.

Biochar, an effective and low-cost amendment for immobilizing heavy metals, has been extensively studied. However, the simultaneous inhibition effects of biochar on the plant uptake for arsenic (As) and cadmium (Cd) in co-contaminated soils are still ambiguous due to their distinct environmental behaviors. A meta-analysis was conducted to quantitatively assess the effects using 1030 individual observations from 52 articles. On average, biochar application significantly decreased the bioavailability of Cd in soils by 50.12%, while slightly increased the bioavailability of As in soils by 2.39%. The more instructive result is that biochar application could also simultaneously reduce the concentration of As and Cd in plants by 25.48% and 38.66%, respectively. The orders of the decreased percentage of As and Cd in various tissues were root < stem< leaf < grain, and root < leaf < stem < grain, respectively. According to the analysis of critical factors, manure biochar, low pyrolysis temperature (at <400 °C), low application rate (<2%), and high SOC (>30 g/kg) were more conducive to reduce the bioaccumulation of As and Cd simultaneously in co-contaminated soils. Pristine and modified biochar could inhibit As and Cd accumulation in crops, but their efficiencies need to be further improved to ensure the safety of crop productions. Overall, the meta-analysis suggests that biochar has the potential to remedy the As and Cd co-contaminated soils.