Iron and sulfur reduction caused by different growth seasons inhibits cadmium transfer in the soil-rice system

Joint utilization of Chinese milk vetch and lime materials mitigates soil cadmium risk and improves soil health in a double-cropping rice system

Cadmium (Cd) in paddy soil poses significant risks to humans due to its strong biological migration and toxicity. Chinese milk vetch (MV) is commonly used as green manure in the paddy fields of southern China and its potential to decrease the availability of Cd has been identified. Nevertheless, the effects of MV combined with lime materials (lime, L; limestone, LS) on Cd availability, soil properties, enzyme activity and comprehensive benefits are still not fully understood in double-cropping rice system. A field study was conducted to investigate these changes. The results indicated that all treatments notably decreased soil available Cd (Avail-Cd) by 19.3-44.3% and 14.9-43.1% during early and late rice, compared with CK. Moreover, the Cd fractions transformed to more stable forms. Compared to CK, all treatments reduced brown rice Cd content by 34.6-64.2% and 12.7-52.5% during the two periods. Furthermore, the translocation factors root to shoot, as well as shoot to brown rice, decreased. The combination led to improvements in soil properties, soil enzyme activity. Meantime, Cd in iron-manganese plaque (IMP) decreased by 31.9-51.1% and 29.0-42.7% respectively during two periods in amendments treatments. Soil pH and DOC were more important factors for Cd bioavailability than other properties. Additionally, rice Cd uptake was positively correlated with Cd in IMP. Enzyme activity exhibited a negative correlation with soil active Cd. Partial Least Squares Path Model (PLS-PM) indicated that the mitigation of Cd pollution helped to improve soil enzyme activity. Grey correlation analysis (GRA) indicated that MVLS showed the best comprehensive benefits in soil-plant system. Overall, the combination of MV and lime materials could reduce Cd availability, enhance soil properties and enzyme activity. And this could be strengthened by the combination. These findings will provide valuable insights for Cd-contaminated soil remediation.

In-situ remediation of cadmium contamination in paddy fields: from rhizosphere soil to rice kernel

Cadmium (Cd) has become an important heavy metal pollutant because of its strong migration and high toxicity. The industrial production process aggravated the Cd pollution in rice fields. Human exposure to Cd through rice can cause kidney damage, emphysema, and various cardiovascular and metabolic diseases, posing a grave threat to health. As modern technology develops, the Cd accumulation model in rice and in-situ remediation of Cd pollution in cornfields have been extensively studied and applied, so it is necessary to sort out and summarize them systematically. Therefore, this paper reviewed the primary in-situ methods for addressing heavy metal contamination in rice paddies, including chemical remediation (inorganic-organic fertilizer remediation, nanomaterials, and composite remediation), biological remediation (phytoremediation and microbial remediation), and crop management remediation technologies. The factors that affect Cd transformation in soil and Cd migration in crops, the advantages and disadvantages of remediation techniques, remediation mechanisms, and the long-term stability of remediation were discussed. The shortcomings and future research directions of in situ remediation strategies for heavily polluted paddy fields and genetic improvement strategies for low-cadmium rice varieties were critically proposed. To sum up, this review aims to enhance understanding and serve as a reference for the appropriate selection and advancement of remediation technologies for rice fields contaminated with heavy metals.

Soil Cd bioavailability response characteristics to microbes in paddy fields with co-incorporation of milk vetch, rice straw and amendments

Excessive heavy metals in soils can threaten food security and soil health. New practical technology is urgently needed to remediate cadmium (Cd) contaminated paddies in many parts of the world. Chinese milk vetch (M), rice straw (R), and soil amendments can reduce Cd activity in soil; however, the mechanism underlying this reduction is not well understood. This study explored the impact of co-incorporation of milk vetch, rice straw, and either lime (L), sesbania biochar (B), or sepiolite on soil Cd bioavailability through field experiments. The results indicated that the rice grain Cd concentrations in soil treated with milk vetch + rice straw + fertilizer (MRF, 16.6 %), milk vetch + rice straw + fertilizer + sesbania biochar (MRFB, 50.1 %), and milk vetch + rice straw + fertilizer + lime (MRFL, 48.3 %) were significantly lower than those in soil treated with fertilizer (F). The acid-soluble Cd concentrations influenced rice grain Cd uptake and were 33.9 % and 47.5 % lower for the MRFB and MRFL treatments, respectively, than for F alone. A decrease in acid-soluble Cd (AciCd) was accompanied by a decrease in Eh and increases in pH, Fe2+, cation exchange capacity, and dissolved organic carbon. The MRFB treatment promoted iron plaque (IP) formation on the rice root surface. The relative abundances of Desulfobacterota and Verrucomicrobiota were higher for the MRFB treatment than for the other treatments. A partial least squares path model confirmed that Aci-Cd and low-crystalline IP (IP-Feh) influenced the rice grain Cd concentration.

Straw removal reduces Cd availability and rice Cd accumulation in Cd-contaminated paddy soil: Cd fraction, soil microorganism structure and porewater DOC and Cd

The impacts of straw removal on rice Cd absorption, behaviour of Cd and microbial community in rhizosphere soil were investigated in paddy fields over two consecutive seasons. The results of the experiments in two fields revealed that straw removal promoted the transformation of soil Cd from acid-extractable and oxidisable fraction to residual fraction and reduced soil DTPA-Cd content with the reduction in DOC and Cd ions in soil porewater, thereby decreasing Cd content in rice. Specifically, the Cd content in brown rice of early rice was below 0.2 mg·kg-1 when all rice straw and roots were removed in the slightly Cd-contaminated soils. The α-diversity of soil microbial communities was less influenced by continuous straw removal, β-diversity was altered and the relative abundances of Anaeromyxobacter, Methylocystis and Mycobacterium microbes were increased. Redundancy analysis and network analysis exhibited that soil pH predominantly influenced the microbial community. Path analysis revealed that the Cd content in brown rice could be directly influenced by the soil Total-Cd and DTPA-Cd, as well as soil pH and OM. Straw removal, including roots removal, is an economical and effective technique to reduce Cd accumulation in rice plants.

Buccal Swab Samples from Japanese Brown Cattle Fed with Limonite Reveal Altered Rumen Microbiome

The areas of the Mount Aso grasslands in Kumamoto, Japan, are the primary location for the breeding of the Kumamoto strain of Japanese Brown cattle (JBRK). Although Aso limonite, deposited by volcanic ash and magma, has been commonly fed to pregnant JBRK in this area, the mechanisms of its salutary effects on pregnant JBRK have not yet been elucidated. Approximately 100 days before the expected day of calf delivery, seven JBRK (four supplemented with limonite and three controls without limonite) were assigned to this study, from which a buccal swab was collected at the highest rumination every 30 days for 90 days. DNA extracted from these swabs was then analyzed using a 16S rRNA gene amplicon sequence analysis. Statistically significant differences between the two groups were discovered through beta-diversity analysis, though results from alpha-diversity analysis were inconclusive. The microbiota identified were classified into six clusters, and three of the main clusters were core-rumen bacteria, primarily cellulose digestion in cluster 1, oral bacteria in cluster 2, and non-core-rumen bacteria in cluster 3. In the limonite group, core-rumen bacteria decreased while non-core-rumen bacteria increased, suggesting that limonite feeding alters rumen microbiota, particularly activation of non-core-rumen microbiota.

Effects of Fe oxides and their redox cycling on Cd activity in paddy soils: A review

Cadmium (Cd) contamination of soils is a global problem, particularly in paddy soils. Fe oxides, as a key fraction of paddy soils, can significantly affect the environmental behavior of Cd, which is controlled by complicated environmental factors. Therefore, it is necessary to systematically collect and generalize relevant knowledge, which can provide more insight into the migration mechanism of Cd and a theoretical basis for future remediation of Cd contaminated paddy soils. This paper summarized that (1) Fe oxides influence Cd activity through adsorption, complexation, and coprecipitation during transformation; (2) compared with the flooded period, the activity of Cd during the drainage period is stronger in paddy soils, and the affinity of different Fe components for Cd was distinct; (3) Fe plaque reduced Cd activity but was associated with plant Fe²⁺ nutritional status; (4) the physicochemical properties of paddy soils have the greatest impact on the interaction between Fe oxides and Cd, especially with pH and water fluctuations.

Mechanism of mercapto-modified palygorskite in reducing soil Cd activity

Mercapto-modified palygorskite (MP) is an efficient novel amendment with superior ability to decrease soil Cd bioavailability, but the unclear immobilization mechanism has become the bottleneck of its performance improvement and precise application. In order to clarify the Cd reducing mechanism of MP, long-term and short-term soil incubation with three types of soils (paddy soil, alluvial soil and yellow mountain soil) and sorption verification experiments were conducted to investigate the dynamic process of soil labile Cd impacted by MP and the synergetic effects on labile Fe, Mn, S and dissolved organic carbon via in-situ diffusive gradients in thin-films and soil solution sampling techniques. MP with four dosages rapidly and continuously decreased soil labile Cd contents by 14.50 % ∼ 89.16 % in long-term incubation, meanwhile low-dosage MP reduced soil labile Fe and Mn contents, but high-dosage MP increased their contents. With MP dosages increased, the effects of Fe-Mn oxides on soil labile Cd content gradually weakened. MP effectively promoted the reduction of Fe adsorbed by clay minerals and enhanced their ability to adsorb Cd. Short-term incubation showed that MP could decline soil labile Cd by 7.17 % ∼ 44.74 %, especially at the dosage 0.4 %. MP was a reduction catalyst to facilitate Fe reduction, which profited for clay minerals adsorbing Cd. The sorption experiments indicated that 0.30 % MP could adsorb 73.34 % Cd²⁺, promote the release of Fe²⁺ from the soil, and stimulate the ability of clay minerals to adsorb Cd. The results revealed that MP decreased soil labile Cd content within 2 d, and MP made soil Cd activity change out of the influence of soil Fe/Mn redox system. The mechanism will be beneficial for the large-scale application of MP in safe utilization of Cd contaminated soil.

The rhizosphere bacterial community contributes to the nutritional competitive advantage of weedy rice over cultivated rice in paddy soil

Background

Weedy rice competes for nutrients and living space with cultivated rice, which results in serious reductions in rice production. The rhizosphere bacterial community plays an important role in nutrient competition between species. It is therefore important to clarify the differences in the diversities of the inter rhizosphere bacterial community between cultivated rice and weedy rice. The differences in compositions and co-occurrence networks of the rhizosphere bacterial community of cultivated rice and weedy rice are largely unknown and thus the aim of our study.

Results

In our study, the different rhizosphere bacterial community structures in weedy rice (AW), cultivated rice (AY) and cultivated rice surrounded by weedy rice (WY) were determined based on 16S rRNA gene sequencing. The majority of the WY rhizosphere was enriched with unique types of microorganisms belonging to Burkholderia. The rhizosphere bacterial community showed differences in relative abundance among the three groups. Network analysis revealed a more complex co-occurrence network structure in the rhizosphere bacterial community of AW than in those of AY and WY due to a higher degree of Microbacteriaceae and Micrococcaceae in the network. Both network analysis and functional predictions reveal that weedy rice contamination dramatically impacts the iron respiration of the rhizosphere bacterial community of cultivated rice.

Conclusions

Our study shows that there are many differences in the rhizosphere bacterial community of weedy rice and cultivated rice. When cultivated rice was disturbed by weedy rice, the rhizosphere bacterial community and co-occurrence network also changed. The above differences tend to lead to a nutritional competitive advantage for weedy rice in paddy soils.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12866-022-02648-1.

Rice Plants (Oryza sativa L.) under Cd Stress in Fe Deficiency Conditions

Due to the environment pollution by cadmium (Cd) near industrial metallurgic factories and the widespread use of phosphorus fertilizers, the problem of toxic Cd effect on plants is well discussed by many authors, but the phytotoxicity of Cd under iron (Fe) deficiency stress has not been sufficiently studied. The aim of the work was to study comprehensively the effect of Cd under Fe deficiency conditions on physiological, biochemical, and anatomical parameters of rice varieties, to identify varietal differences in plant response to the effect of double stress. Relative resistance and sensitivity to the joint effect of Cd and Fe deficiency stress rice varieties have been identified. Double stress decreased a linear growth and biomass accumulation of roots and shoots (by 36-50% and 33-46% and 32-56% and 32-48%, accordingly), content of photosynthetic pigments (Chla, Chlb, and carotenoids by 36-51%, 32-47%, and 64-78%, accordingly), and relative water content (by 18-26%). Proline content increased by 28-103% in all rice varieties, but to a lesser extent in sensitive varieties. The thickness of the lower and upper epidermis and the diameter of vascular bundles of leaves decreased by 18-50%, 46-60%, and 13-48%, accordingly. The thickness of the root endodermis and exodermis and diameter of the central cylinder mainly decreased. The thickness of the exodermis increased slightly by 7%, and the diameter of the central cylinder remained at the control level in resistant Madina variety while in sensitive Chapsari variety, these indicators decreased significantly by 50 and 45%, accordingly. Thus, the aggravation of adverse effect of Cd under Fe deficiency conditions and the varietal specificity of plants' response to double stress were shown. It creates the need for further study of these rice varieties using Fe to identify mechanisms for reducing the toxic effect of Cd on plants as well as the study of Fe and Cd transporter genes at the molecular level.