Cadmium (Cd) and arsenic (As) contamination of soil has been a public concern due to their potential accumulation risk through the food chain. This study was conducted to investigate the performance of ferrous sulfate (FeSO₄) and ferric oxide (Fe₂O₃) nanoparticle (Nano-Fe) to stabilize the concentrations of Cd and As in paddy soil. Both Fe treatments led to low extractable Cd and the contents of specifically sorbed As contents, increased (p < 0.05) the Shannon index and decreased (p < 0.05) the Simpson diversity indices compared with the control. Nano-Fe increased the relative abundances of Firmicutes and Proteobacteria and decreased the abundances of Acidobacteria and Chloroflexi. Moreover, the addition of both forms of Fe promoted the formation of Fe plaque and decreased the translocation factor index (TFs) _root/soil, TFs _shoot/root, and TFs _grain/shoot of Cd and As. These results suggest that exogenous Fe may modify the microbial community and decrease the soil available Cd and As contents, inhibit the absorption of Cd and As by the roots and decrease the transport of Cd and As in rice grains and the risk intake in humans. These findings demonstrate that soil amendment with exogenous Fe, particularly Nano-Fe, is a potential approach to simultaneously remediate the accumulation of Cd and As from the soil to rice grain systems.

The intercropping of arsenic (As) hyperaccumulator Chinese brake fern (Pterisvittata L.) with maize (Zea mays L.) is being widely utilized to enhance phytoremediation without impeding agricultural production. Arbuscular mycorrhizal (AM) fungi can regulate the physiological and molecular responses of plants in tolerating heavy metal stress. We studied the effects of inoculation with AM fungi on As uptake by maize and P. vittata grown in soil contaminated with As. The results show that infection with the fungus Claroideoglomus etunicatum (Ce) increased the biomass of maize and P. vittata. Moreover, infection with Ce significantly reduced the accumulation of As and the coefficient for root–shoot transport of As in maize, whereas it enhanced the accumulation of As and coefficient for root–shoot transport of As in P. vittata. Infection with Ce led to a high content of available As in the soil planted with P. vittata, while there was a lower content of available As in the soil planted with maize. The different concentrations of available As in the soils suggest that inoculation with Ce may enhance the secretion of organic acids, particularly citric acid and tartaric acid, by maize roots and promote rhizosphere acidification, which then causes a decrease in As uptake by maize. Inoculation with Ce decreased the secretion of citric acid from P. vittata and promoted rhizosphere alkalization, which then caused an increase in As uptake by P. vittata and maize. Thus, co-combining AM fungi in the intercropping of the hyperaccumulator P. vittata with maize could be a promising approach to improving the efficiency of remediating As-contaminated soil.

Arbuscular mycorrhizal fungi (AMF) have been widely shown to significantly promote the growth and recovery of Pteris vittata L. growth and repair under arsenic stress; however, little is known about the molecular mechanisms by which AMF mediate the efficient uptake of arsenic in this species. To understand how AMF mediate P. vittata arsenic metabolism under arsenic stress, we performed P. vittata root transcriptome analysis before and after Claroideglomus etunicatum (C. etunicatum) colonization. The results showed that after C. etunicatum colonization, P. vittata showed greater arsenic resistance and enrichment, and its dry weight and arsenic accumulation increased by 2.01–3.36 times. This response is attributed to the rapid reduction and upward translocation of arsenic. C. etunicatum enhances arsenic uptake by mediating the MIP, PHT, and NRT transporter families, while also increasing arsenic reduction (PvACR2 direct reduction and vesicular PvGSTF1 reduction). In addition, it downregulates the expression of ABC and P-type ATPase protein families, which inhibits the compartmentalization of arsenic in the roots and promotes its translocation to the leaves. This study revealed the mechanism of C. etunicatum-mediated arsenic hyperaccumulation in P. vittata, providing guidance for understanding the regulatory mechanism of P. vittata.

The national soil risk screening values (RSVs) and intervention values (RIVs) have limited applicability in karst regions with high geochemical backgrounds, often leading to unnecessary remediation efforts. To address this issue, 125 paired soil–rice samples from Gejiu–Mengzi, Yunnan Province, China, were analyzed. We found that the RSV misclassification rates (mainly false positives) for cadmium (Cd) and arsenic (As) in neutral-to-alkaline paddies (pH > 6.5) ranged from 52.0–66.4%. Soil iron/manganese (Fe/Mn) oxides and pH were identified as critical factors controlling Cd/As bioavailability in rice. This decoupling is primarily governed by soil alkalinity and abundant Fe/Mn oxides, which immobilize metals despite high total concentrations. This also explains why there is such a high rate of false positives. By integrating multiple linear regression and species sensitivity distribution (SSD) models, we derived regional safety thresholds (STs) and hazard thresholds (HTs). The newly established STs (Cd: 4.54–7.12 mg·kg ^-1 ; As: 91.43–92.30 mg·kg ^-1 ) were significantly higher than the national RSVs (Cd: 0.6–0.8 mg·kg ^-1 ; As: 20–25 mg·kg ^-1 ). The HT for Cd (9.66–10.12 mg·kg ^-1 ) was 2.5–3.2 times greater than the RIV for Cd, whereas the HT for As (96.90–97.15 mg·kg ^-1 ) was slightly lower than the RIV for As (100–120 mg·kg ^-1 ). Applying the ST–HT system increased the soil quality assessment accuracy from 28.1–95.6% to 71.8–100%, drastically reducing the need for unnecessary remediation. This study not only serves as a region-specific framework for safe rice cultivation in karst areas of Yunnan, but also highlights the potential for integrating soil–plant system bioavailability with physiological tolerance mechanisms. Furthermore, we provide a crucial benchmark for evaluating innovative remediation strategies aimed at alleviating metal stress in plants.