Cadmium (Cd) contamination in paddy soil often results in elevated Cd concentrations in rice grain, which is a serious concern threatening food safety. Most of the Cd accumulated in rice grain is derived from its remobilization in paddy soil during the grain filling period when paddy water is drained. We have previously shown that the voltaic cell effect controls the oxidative release of cadmium sulfide (CdS) during the drainage period. Metal sulfides with lower electrochemical potentials than CdS can suppress the oxidation of CdS. In the present study, we tested whether amendments of ZnSO₄ or MnSO₄ could enhance the suppressive voltaic effect on Cd release and subsequent accumulation in rice grain. The one-time addition of ZnSO₄ (75 kg/ha Zn) decreased CaCl₂-extractable Cd concentrations in soils by 32–64% in pot experiments and by 16–30% in field trials during the drainage period. Consequently, Cd concentrations in brown rice were reduced by 74–87% and 60–72% in pot experiments and field trials, respectively. Importantly, this effect persisted in the second year without further addition. The amendment of MnSO₄ had similar effects in decreasing soil extractable Cd and Cd concentrations in brown rice. These effects were not attributed to the addition of sulfate. A single application of such doses of ZnSO₄ or MnSO₄ (e.g. 75–150 kg/ha Zn or Mn) only caused a marginal increase in soil Zn or Mn concentrations and had no significant impact on grain yield. Taken together, amendments of ZnSO₄ and/or MnSO₄ (at the rate of 75–150 kg/ha Zn and or Mn) formed a protective voltaic cell partner against the oxidative dissolution of CdS and thus were highly effective in reducing Cd accumulation in rice grain. This work provides a simple but effective method to decrease soil Cd availability during soil drainage and mitigate Cd accumulation in rice to ensure food safety.

Both inorganic and organic fertilizers are widely used to increase rice yield. However, these fertilizers are also found to aggravate mercury methylation and methylmercury (MeHg) accumulation in paddy fields. The aim of this study was to reveal the mechanisms of inorganic and organic fertilizers on MeHg accumulation in rice grains, which are not yet well understood. Potting cultures were conducted in which different fertilizers were applied to a paddy soil. The results showed that both inorganic and organic fertilizers increased MeHg concentrations rather than biological accumulation factors (BAFs) of MeHg in mature rice grains. Inorganic fertilizers, especially nitrogen fertilizer, enhanced the bioavailability of mercury and the relative amount Hg-methylating microbes and therefore intensified mercury methylation in paddy soil and MeHg accumulation in rice grains. Unlike inorganic fertilizers, organic matter (OM) in organic fertilizers was the main reason for the increase of MeHg concentrations in rice grains, and it also could immobilize Hg in soil when it was deeply degraded. The enhancement of MeHg concentrations in rice grains induced by inorganic fertilizers (5.18–41.69%) was significantly (p < 0.05) lower than that induced by organic fertilizers (80.49–106.86%). Inorganic fertilizers led to a larger increase (50.39–99.28%) in thousand-kernel weight than MeHg concentrations (5.18–41.69%), resulting in a dilution of MeHg concentrations in mature rice grains. Given the improvement of soil properties by organic fertilizer, increasing the proportion of inorganic fertilizer application may be a better option to alleviate MeHg accumulation in rice grains and guarantee the rice yield in the agricultural production.

The stable stabilization of uranium (U) in iron (Fe) containing environments is restricted by the reoxidation of UO₂. In the current study, based on air reoxidation tests, we propose a novel two steps accumulation method to enrich microbial consortia from paddy soil. The constructed microbial consortia, denoted as the Fe–U bacteria, can co-precipitate U and Fe to form stable Fe–U solids. Column experiments running for 4 months demonstrated the production of U(IV)–O–Fe(II) precipitates containing maximum of 39.51% uranium in the presence of Fe–U bacteria. The reoxidation experiments revealed the U(IV)–O–Fe(II) precipitates were more stable than UO₂. 16S rDNA high throughput sequencing analysis demonstrated that Acinetobacter and Stenotrophomonas were responsible for Fe and U precipitation, while, Caulobacteraceae and Aminobacter were crucial for the formation of U(VI)-PO₄ chemicals. The proposed two steps accumulation method has an extraordinary application potential in stable immobilization of uranium in iron containing environments.

As a redox-sensitive element, manganese (Mn) plays a critical role in Cd mobilization, especially in paddy soil. In an anoxic environment, the precipitation of Mn(II)-hydroxides specifically favors Cd retention, while draining the paddy fields results in substantial remobilization of Cd. However, how the change in Mn redox states during the periodical transit of anoxic to oxic systems affects Cd mobility remains unclear. In this study, we demonstrate that the radical effect generated during the oxidation of Mn(II)-hydroxides exerts a significant effect on the oxidative dissolution of Cd during the aeration of paddy soils. The extractable Cd concentration decreased rapidly during the reduction phases but increased upon oxidation, while Cd availability produced the opposite effect with soil pe + pH and the extractable Mn concentration. Inhibiting the oxidation of Mn(II)-containing phases by microbes suppressed the production of hydroxyl free radicals (•OH) and Cd mobilization in the drainage phase. Analysis of X-ray absorption spectroscopy and sequential extraction demonstrated that the transformation from the Mn phase of Mn(II) to Mn(III/IV) determines Cd solubility. Altogether, the oxidization of Mn(II)-hydroxides was associated with the generation of significant amounts of •OH. The dissolution of Mn(II)- incorporating phases lead to a net release of Cd into soils during soil aeration.

Soil microorganisms represent one of the largest biodiversity reservoirs. However, most low-abundance, slow-growing or dormant microorganisms in soils are difficult to capture with traditional enrichment culture methods. These types of microorganisms represent a valuable “microbial seed bank”. To better exploit and utilize this “microbial dark matter”, we developed a novel strategy that integrates single-cell-level isolation with microfluidics technology and culture with resuscitation-promoting factor (Rpf) to isolate biphenyl-degrading bacteria from four typical soils (paddy soil, red soil, alluvial soil and black soil) in eastern China. Multitudinous bacteria were successfully isolated and cultured; some of the identified clades have not been previously linked to biphenyl biodegradation, such as Actinotalea, Curtobacterium and Rothia. Soil microcosmic experiments validated that some bacteria are responsible for biphenyl degradation in soil. In addition, genomic sequencing and Illumina MiSeq sequencing of 16S rRNA genes indicated that exogenous Rpf mainly promotes the recovery and growth of bacteria containing endogenous Rpf-encoding genes. In summary, this study provides a novel strategy for capturing target functional microorganisms in soils, indicates potential bioresources for the bioremediation of contaminated soils, and enhances our current understanding of the mechanisms involved in the response to exogenous Rpf.

This study examined the effect of tetracycline addition on arsenic (As) mobilization and biotransformation in two contrasting soils (upland soil and paddy soil) under flooded conditions. The soils with added tetracycline (0–50 mg kg⁻¹) were incubated for 30 days, and soil properties and microbial functional genes over time were quantified. Tetracycline significantly promoted As reduction and As release into porewater in both soils. The enhancement had resulted from an increase in the concentration of dissolved organic carbon and a decrease in soil redox potential. Tetracycline also increased the abundances of As-reducing genes (arsC and arrA) and the relative abundances of As-reducing bacteria Streptomyces, Bacillus, Burkholderia, Clostridium and Rhodococcus, all of which have been found resistant to tetracycline. These genera play a key part in stimulating As reduction in the presence of tetracycline. The study indicated the significance of tetracycline in the biochemical behavior of As in flooded soils and provided new insights into the potential effects of tetracycline on the quality and safety of agricultural products in the future.

In this study, we assessed the impact of zinc oxide (ZnO) and iron oxide (FeO) (<36 nm) nanoparticles (NPs) as well as their sulphate salt (bulk) counterpart (0, 25, 100 mg/kg) on rice growth and seed quality as well as the microbial community in the rhizosphere environment of rice. During the rice growing season 2021–22, all experiments were conducted in a greenhouse (temperature: day 30 °C; night 20 °C; relative humidity: 70%; light period: 16 h/8 h, day/night) in rice field soil. Results showed that low concentrations of FeO and ZnO NPs (25 mg/kg) promoted rice growth (height (29%, 16%), pigment content (2%, 3%)) and grain quality parameters such as grains per spike (8%, 9%), dry weight of grains (12%, 14%) respectively. As compared to the control group, the Zn (2%) and Fe (5%) accumulations at their respective low concentrations of NP treatments showed stimulation. Interestingly, our results showed that at low concentration of both the NPs the soil microbes had more diversity and richness than those in the bulk treated and control soil group. Although a number of phyla were affected by the presence of NPs, the strongest effects were observed for change in the abundance of the three phyla for Proteobacteria, Actinobacteria, and Planctomycetes. The rhizosphere environment was notably enriched with potential streptomycin producers, carbon and nitrogen fixers, and lignin degraders with regard to functional groups of microorganisms. However, microbial communities mainly responsible for chitin degradation, ammonia oxidation, and nitrite reduction were found to be decreased. The results from this study highlight significant changes in several plant-based endpoints, as well as the rhizosphere soil microorganisms. It further adds information to our understanding of the nanoscale-specific impacts of important micronutrient oxides on both rice and its associated soil microbiome.

Mixed pollution due to heavy metals (HMs), especially cadmium (Cd), lead (Pb), and arsenic (As), seriously endangers the safety of food produced in paddy soil. In the field experiments, foliar application of 2,3-dimercaptosuccinic acid (DMSA) at the flowering stage was found to significantly reduce the levels of Cd, Pb, total As, and inorganic As (iAs) in rice grains by 47.95%, 61.76%, 36.37%, and 51.24%, respectively, without affecting the concentration of metallonutrients, including Mn, K, Mg, Ca, Fe, and Zn. DMSA treatment significantly reduced the concentrations of Cd, Pb, and As in the panicle node, panicle neck, and rachis, while those in the flag leaves were significantly increased by up to 20.87%, 49.40%, and 32.67%, respectively. DMSA application promoted the transport of HM from roots and lower stalks to flag leaves with a maximum increase of 34.55%, 52.65%, and 46.94%, respectively, whereas inhibited the transport of HM from flag leaves to panicle, rachis, and grains. Therefore, foliar application of DMSA reduced Cd, Pb, and As accumulation in rice grains by immobilizing HMs in flag leaves. Thus, this strategy could act as a promising agronomic measure for the remediation of mixed HM contamination in paddy fields.

Biochar is often applied to paddy soils as a soil improver, as it retains nutrients and increases C sequestration; as such, it is a tool in the move towards C-neutral agriculture. Nitrogen (N) fertilizers have been excessively applied to rice paddies, particularly in small farms in China, because N is the major limiting factor for rice production. In paddy soils, dynamic changes in iron (Fe) continuously affect soil emissions of methane (CH₄) and carbon dioxide (CO₂); however, the links between Fe dynamics and greenhouse gas emissions, dissolved organic carbon (DOC), and rice yields following application of biochar remain unclear. The aims of this study were to examine the effects of two rates of nitrogen (N)-enriched biochar (4 and 8 t ha⁻¹ y⁻¹) on paddy soil C emissions and storage, rice yields, and Fe dynamics in subtropical early and late rice growing seasons. Field application of N-enriched biochar at 4 and 8 t ha⁻¹ increased C emissions in early and late rice, whereas application at 4 t ha⁻¹ significantly increased rice yields. The results of a culture experiment and a field experiment showed that the application of N-enriched biochar increased soil Fe²⁺concentration. There were positive correlations between Fe²⁺concentrations and soil CO₂, CH₄, and total C emissions, and with soil DOC concentrations. On the other way around, these correlations were negative for soil Fe³⁺concentrations. In the soil culture experiment, under the exclusion of plant growth, N-enriched biochar reduced cumulative soil emissions of CH₄ and CO₂. We conclude that moderate inputs of N-rich biochar (4 t ha⁻¹) increase rice crop yield and biomass, and soil DOC concentrations, while moderating soil cumulative C emissions, in part, by the impacts of biochar on soil Fe dynamics. We suggest that water management strategies, such as dry-wet cycles, should be employed in rice cultivation to increase Fe²⁺ oxidation for the inhibition of soil CH₄ and CO₂ production. Overall, we showed that application of 4 t ha⁻¹ of N-enriched biochar may represent a potential tool to improve sustainable food production and security, while minimizing negative environmental impacts.

Cadmium (Cd) and lead (Pb) are toxic heavy metals that impact human health and biodiversity. Removal of Cd/Pb from contaminated soils is a means for maintaining environmental sustainability and biodiversity. In this study, we applied a newly modified material fly ash (NA), zeolite (ZE), and fly ash (FA) to the paddy soils and evaluated the effects of Cd/Pb accumulation in rice via a one-year field experiment. The results showed that the application of NA and ZE enhanced the soil pH and nutrients to a large extent and reduced the availability of Cd/Pb in soil. The Cd and Pb concentrations in rice grains decreased by 32.8% and 62.9%, respectively, with the NA treatments. Similarly, the application of ZE reduced the Cd and Pb concentrations in rice grains by a factor of 27.9% and 63.5%, respectively, which indicates that the amendments can promote the transfer of Cd and Pb from acid-exchangeable fraction to oxidizable and residual fractions. The Cd/Pb showed a significant positive correlation to other metal ions and a negative correlation to the nutrients. Generally, the application of NA and ZE was effective in reducing Cd/Pb accumulation and improving rice yield. Moreover, the NA was more cost-effective than ZE. Hence, this study proves that NA may be a better amendment for remediation of Cd/Pb contaminated soils.