Water management such as drainage for creating aerobic conditions is considered to be an adequate method for reducing the accumulation of arsenic (As) in rice grains; however, it is difficult to conduct drainage operations in some areas that experience a lengthy rainy season as well as in soils with poor drainage. In this regard, application of oxygen-releasing compounds (ORCs) may be an alternative method for maintaining aerobic conditions even under flooding in paddy soils. Therefore, a pot experiment was conducted to investigate the effects of application of an ORC, calcium peroxide (CaO₂), on the growth and accumulation of As in rice plants grown in As-contaminated paddy soils. The rice plants were grown in two soils with different characteristics and As levels, and all of the tested soils were treated with 0, 5, 10, and 20 g CaO₂ kg⁻¹. Results revealed that the concentration of As and the distribution of arsenite in the pore water of all tested soils was reduced by CaO₂ application. In addition, the grain yields increased and the concentration of inorganic As in brown rice decreased by 25–45% upon CaO₂ treatment of low-As-level soils (<16 mg kg⁻¹). However, the effect of CaO₂ application on the accumulation of inorganic As in brown rice in As-enriched soils (>78 mg kg⁻¹) could not found in this study, due to the rice plant suffered from serious As phytotoxicity. It suggests that CaO₂ amendment may be suitable for reducing the As concentration of rice grains grown in low-As-level paddy soils, but for As-enriched soils, the proposed CaO₂ application method is not feasible.

Environmental complexity leads to differences in the spatial distribution of heavy metal pollution in soil and rice. Such spatial differences will seriously affect the safety of planted rice and can impact regional management and control. How to scientifically reveal these spatial differences is an urgent problem. In this study, the spatial mismatch relationship between Cd pollution in soil and rice grains (brown rice) was first explored by the interpolation method. To further reveal the causes of these, the specific recognition rules of the spatial relationship of Cd pollution were extracted based on a decision tree model, and the results were mapped. The results revealed a spatial mismatch in Cd pollution between the soil and rice grains in the study area, and the main results are as follows: (i) slight soil pollution and safe rice accounted for 68.88% of the area; (ii) slight soil pollution and serious rice pollution accounted for 13.39% of the area and (iii) safe soil and serious rice pollution accounted for 11.63% of the area. In addition, 11 recognition rules of Cd spatial pollution relationship between soil and rice were proposed, and the main environmental factors were determined: SOM (soil organic matter), Dis-residence (distance from residential area), soil pH and LAI (leaf area index). The average accuracy of rule recognition was 75.90%. The study reveals the spatial mismatch of heavy metal pollution in soil and crops, providing decision-making references for the spatial accurate identification and targeted prevention of heavy metal pollution spaces.

The contamination of arsenic (As) and cadmium (Cd) in paddy soils is widely reported and these two metals are difficult to be co-remediated due to the contrasting chemical behaviors. This poses a challenge to simultaneously decrease their availability in soil and accumulation in rice via immobilization by amendments, especially in in-situ fields. This study compared the effects of carbide slag, lodestone and biochar on the bioavailability of As and Cd in soil and their accumulation in rice tissues and root Fe–Mn plaque at tillering and mature stages in a paddy field. The addition of three amendments significantly limited the mobilization of As and Cd in soil and decreased their accumulations in brown rice by 30–52% and 9–21%, respectively. Carbide slag was most whereas lodestone least effective in As and Cd immobilization in the tested contaminated soils. Community Bureau of Reference (BCR) sequential extraction analysis showed that the amendments changed the forms of As and Cd to less-available. Activated functional groups of the amendments (e.g. –OH, C–O, OC–O, OH⁻ and CO₃²⁻) sequestered metals by precipitation, adsorption, ion exchange or electrostatic attributes contributed greatly to the As and Cd immobilization in soil. Furthermore, the amendments promoted the formation of Fe–Mn plaque in rice roots, which further limited the mobility of As and Cd in soil and prevented their transport from soil to rice roots. The application of carbide slag and biochar but not lodestone increased rice yield compared to the unamended control, indicating their applicability in situ remediation. Our study gives a strong reference to select immobilizing amendments for food safe production in co-contaminated paddy soils.

Rice is a main source of dietary cadmium (Cd), thus, how to reduce the Cd concentration in brown rice has received extensive attention worldwide. In three acidic paddy soils slightly to moderately contaminated with Cd, a series of field experiments were conducted to evaluate the effects of different proportions of nitrogen-phosphorus-potassium (N-P-K) fertilizer (urea, calcium magnesium phosphate, and potassium carbonate, respectively) alone or coupled with a topdressing of manganese (Mn) fertilizer at the tillering stage on reducing Cd bioavailability in soil and uptake in rice. The rational application of N-P-K fertilizer not only provided the basic nutrients to promote the normal growth of rice but also increased soil pH and thereby reduced the Cd bioavailability in soil. The Mg(NO₃)₂-extracted Cd concentrations in the three soils were reduced by 26.46–56.53%, while TCLP-extracted Cd were reduced by 19.87–45.41%, with little influence on soil cation exchange capacity (CEC) and organic matter (OM). The application of Mn fertilizer at the tillering stage increased Mn and Cd sequestration in the iron plaque. The Mn content in iron plaque increased by 15.71–58.67% and a significant positive correlation between Cd and Mn was observed at the three sites. Collectively, this combined method of fertilization significantly reduced Cd accumulation in rice tissues, the Cd concentrations in roots of treated plants decreased by 11.18–37.78%, whereas the concentrations in straw decreased by 13.16–41.03%. Particularly to brown rice, in which accumulation decreased by 25.19–44.70%, 37.35–47.84%, and 38.00–60.88% in three typical paddy fields, but no significant effect was observed for the Cd translocation factors (TF) among rice tissues. Thus, the basal application of combined urea and alkaline inorganic fertilizers followed by topdressing of Mn fertilizer may be a promising and cost-effective tactics for the remediation of Cd-contaminated paddy soils.

In situ immobilization of heavy metals in contaminated soils using industrial by-products is an attractive remediation technique. In this work, titanium gypsum (TG) was applied at two levels (TG-L: 0.15% and TG-H: 0.30%) to simultaneously reduce the uptake of cadmium (Cd), lead (Pb) and arsenic (As) in rice grown in heavy metal contaminated paddy soils. The results showed that the addition of TG significantly decreased the pH and dissolved organic carbon (DOC) in the bulk soil. TG addition significantly improved the rice plants growth and reduced the bioavailability of Cd, Pb and As. Particularly, bioavailable Cd, Pb and As decreased by 35.2%, 38.1% and 38.0% in TG-H treatment during the tillering stage, respectively. Moreover, TG application significantly reduced the accumulation of Cd, Pb and As in brown rice. Real-time PCR analysis demonstrated that the relative abundance of sulfate-reducing bacteria increased with the TG application, but not for the iron-reducing bacteria. In addition, 16S rRNA sequencing analysis revealed that the relative abundances of heavy metal-resistant bacteria such as Bacillus, Sulfuritalea, Clostridium, Sulfuricella, Geobacter, Nocardioides and Sulfuricurvum at the genus level significantly increased with the TG addition. In conclusion, the present study implied that TG is a potential and effective amendment to immobilize metal(loid)s in soil and thereby reduce the exposure risk of metal(loid)s associated with rice consumption.

The accumulation of methylmercury (MeHg) in rice is an important MeHg exposure pathway in humans in several mercury (Hg)-contaminated areas. In this study, the effects of low-dose biochar (BC) amendment (0.3%, w/w) on MeHg mobility/phytoavailability in different Hg-contaminated paddy soils, MeHg accumulation in rice plants and the health risks associated with MeHg-laden rice consumption were investigated. Soils amended with different doses of bamboo-derived BC (0.3, 0.5, and 1%, w/w) were incubated under anoxic conditions in microcosm experiments. In addition, pot experiments were conducted involving rice cultivation with a low BC application rate (0.3%, w/w). We observed that (1) the fraction of extractable MeHg in soils decreased with BC addition in both the microcosm and pot experiments; (2) MeHg concentrations in the rice grains (brown rice) significantly decreased by 56–88% in response to BC amendment, which may be attributed mainly to decreases in MeHg mobility/phytoavailability in the soil; and (3) the hazard quotient (HQ) values for adults and children and fetal intelligence quotient (IQ) decrements associated with MeHg-laden rice consumption were significantly alleviated under BC amendment. Taken together, our findings suggest that a low dose of BC (0.3%, w/w) could have great potential for mitigating the health risks of dietary MeHg exposure from the consumption of rice grown in mercury (Hg)-contaminated areas.

In order to unravel the cadmium (Cd) enrichment patterns in rice (Oryza sativa L.) grown under different exogenous exposure pathways, the pot experiment was conducted in a greenhouse. Cd was added to the soil-rice system via mixing soil with Cd-containing solution, irrigating the pots with Cd-containing water and leaf-spraying with Cd solution to simulate soil pollution (SPS), irrigation water pollution (IPS), and atmospheric deposit pollution sources (APS), respectively. No significant (p > 0.05) differences in plant height and rice grain yield were observed among all treatments including three different Cd pollution sources and control. The contents of Cd in rice plants significantly (p < 0.05) increased with increase in Cd concentrations in three pollution sources. The distribution pattern of Cd in the rice plant organs treated with SPS and IPS followed the order: roots > stems > leaves > husk > brown rice, while it was leaves > roots > stems > husk > brown rice treated with APS. At the same level of treatment, the highest concentration of Cd was observed in rice organs (except for middle and high concentrations treatment roots) grown under APS, followed by IPS and SPS, suggesting that the Cd bioavailability from different pollution sources followed the order of APS > IPS > SPS. It is concluded that the atmospheric pollution contributed more enrichment of rice with Cd. Therefore, in field environment, air deposits should also be analyzed for toxic metals during assessment of food chain contamination and health risk.

Large areas of soils in China are contaminated with Cd and are deficient in Se. Therefore, here, we aimed to reduce Cd accumulation while increasing Se content in rice grain, and to elucidate the mechanisms associated. A greenhouse pot experiment was conducted to determine grain concentrations of Se and Cd upon foliar spraying of Se combined with the application of horse manure and/or fly ash to different contaminated soils containing Cd 0.51 (T1), 1.46 (T2), and 4.59 mg Cd kg⁻¹ (T3). The amount of Fe, Si, and Cd in root iron plaque, and concentrations of Cd and Si in rice tissues were also determined. Foliar spray of Se increased Se concentration in brown rice from approximately 0.04 to 0.15 mg kg⁻¹. Fly ash significantly reduced Cd concentration in brown rice from 0.07 to 0.05, 0.15 to 0.09, and 1.00 to 0.55 mg kg⁻¹ at the T1, T2, and T3 treatment levels, respectively, and soil Cd bioavailability (by at least 33.3%), while it increased Si content in rice roots and shoots by at least 34%. The increase of Si concentration in rice tissues inhibited Cd translocation to brown rice by at least 17%. Horse manure increased the formation of root Fe plaque by approximately 2.3-fold, which resulted in the significant reduction of Cd accumulation in brown rice, shoots, and roots by 36–56%. Thus, foliar spray of Se in combination with the application of fly ash and horse manure proved an effective method to produce Cd-low and Se-rich rice.

Paddy soil contamination by cadmium (Cd) and arsenic (As) is a great concern. Field experiments were conducted to study the effects of steel slag (SS, 2.0 and 4.0 t ha⁻¹) on the solubility of Cd and As in soil and their accumulation by rice plants grown in a historically co-contaminated paddy field with Cd and As. The results showed that SS amendment (4.0 t ha⁻¹) significantly decreased soluble concentrations of Cd in pore-water but increased that of As, related to markedly elevated soil pH and soluble silicon, phosphorus of pore-water in rice rhizosphere at both heading and mature stages. The amendments also evidently decreased Cd but enhanced As in iron plaque on root surfaces, while the formation of iron plaque was not significantly increased. Further, SS amendment (4.0 t ha⁻¹) markedly reduced Cd concentrations in rice tissues (roots, straw, and brown rice) by 48–78% at both stages, though increased As by 13–38%. Cadmium translocation from roots to aerial parts decreased significantly after the amendments, but not for As. Besides, SS application increased the biomass of roots, straw and grains, and root antioxidant enzyme activities. Collectively, steel slag decreased Cd accumulation in rice tissues and in iron plaque but increased those of As, likely due to steel slag decreasing soluble Cd and enhancing soluble As in pore-water, related to soil pH and soluble nutrients (Si, P), and restraining Cd translocation within rice. Our results indicate that steel slag represents a favorable potential for Cd-contaminated paddy soils, though it seems undesirable for Cd and As co-contamination.

The iron (Fe) (hydro)oxides deposited around rice roots play an important role in arsenic (As) sequestration in paddy soils, but there is no systematic study on the relative importance of Fe (hydro)oxides on root surface and in rhizosphere soil in limiting As bioavailability. Twenty-seven rice genotypes were selected to investigate effects of Fe (hydro)oxides on As uptake by rice in an alkaline paddy soil. Results indicated that the As content was positively correlated with the Fe content on root surface, and most of As (88–97%) was sequestered by poorly crystalline and crystalline Fe (hydro)oxides in the alkaline paddy soil. The As sequestration by Fe (hydro)oxides on root surface (IASᵣₒₒₜ 16.8–25.0 mg As/(g Fe)) was much higher than that in rhizosphere (IASᵣₕᵢzₒ 1.4–2.0 mg As/(g Fe)); therefore, in terms of As immobilization, the Fe (hydro)oxides on root surface were more important than that in rhizosphere. However, the As content in brown rice did not have significant correlation with the As content on root surface but was significantly correlated (R² = 0.43, P < 0.05) with the partition ratio (PRAₛ = IASᵣₒₒₜ/IASᵣₕᵢzₒ) of As sequestration on root surface and in rhizosphere, which suggested that Fe (hydro)oxides on root surface did not play the controlling role in lowering As uptake, and the partition ratio PRAₛ would be a better indicator to evaluate effects of Fe (hydro)oxides around roots on As uptake by rice.