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.

The alkaline mineral amendment is a practical means of alleviating Cd concentration in rice grain (CdR) in the short-term; however, the long-term remediation effect of mineral amendment on the CdR and the eco-environmental controls remains unknown. Here a mineral (Si–Ca–Mg) amendment, calcined primarily from molybdenum tailings and dolomite, was applied biannually over 6 years (12 seasons) to acidic and moderately Cd-contaminated double-rice cropping ecosystems. This study investigated the inter-annual variation of Cd in the rice-soil ecosystem and the eco-environmental controls in subtropical rice ecosystems. CdR was reduced by 50%–86% following mineral amendment. The within-year reduction in CdR was similar between early rice (50%–86%, mean of 68%) and late rice (68%–85%, mean of 74%), leading to CdR in all early rice and in 83% of late rice samples below the upper limit (0.2 mg kg⁻¹) of the China National Food Safety Standards. In contrast, the inter-annual reduction in CdR was moderately variable, showing a greater CdR reduction in the later 3 years (73%–86%) than in the former 3 years (54%–79%). Three years continuous mineral amendment was required to guarantee the safety rice production. The concentrations of DTPA-extractable and exchangeable Cd fractions in soil were reduced, while the concentration of oxides-bound Cd was increased. In addition, the soil pH, concentrations of Olsen-P and exchangeable Ca and Mg were elevated. These imply a lower apparent phytoavailability of Cd in the soil following mineral amendment. An empirical model of the 3-variable using soil DTPA-Cd, soil Olsen-P, and a climatic factor (precipitation) effectively predicted temporal changes in CdR. Our study demonstrates that Cd phytoavailability in soil (indexed by DTPA-extractable Cd) and climatic factors (e.g., temperature and precipitation) may directly/indirectly control the inter-annual reduction in CdR following mineral amendment in slightly and moderately Cd-contaminated paddy ecosystems.

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.

Hydrochar (HC), an environment-friendly material, enhances soil carbon sequestration and mitigate greenhouse gases (GHGs) emissions in croplands. In this study, the water-washed HC (WW-HC) was applied to paddy soil to investigate effects on nitrous oxide (N₂O) and methane (CH₄) emissions during rice growth period. Four treatments, namely control (without N fertilizer and WW-HC), N fertilizer (WW-HC00), N fertilizer with 0.5 wt% WW-HC (WW-HC05) and N fertilizer with 1.5 wt% WW-HC (WW-HC15), were established. Results showed the WW-HC addition reduced N₂O and CH₄ emissions, global warming potential (GWP) and greenhouse gas intensity (GHGI) during the growing season. Moreover, the WW-HC application reduced N₂O cumulative emission (P < 0.05) (by 28.6% and 23.8% for WW-HC05 and WW-HC15, respectively). It was mainly due to the reduced ratio of (nirK + nirS) to nosZ under WW-HC15 (P < 0.05). Compared with WW-HC00, the WW-HC05 reduced CH₄ cumulative emissions by 14.8%, while the WW-HC15 increased by 9.7%. This might be ascribed to the significantly reduced expression of the methanogenic mcrA gene and ratio of mcrA to pmoA by WW-HC (P < 0.05). The WW-HC05 amendment decreased GWP and GHGI by 18.6% and 32.5%, respectively. Furthermore, the WW-HC application greatly improved nitrogen use efficiency by 116–145% compared with the control. Our study indicates the WW-HC application is a promising GHGs mitigation practice in paddy fields.

Arsenic (As) has been recognized as one of the most toxic metalloids present in the surface soil contaminating food chain and posing threat to human life. Sulfur (S) fertilizer is often supplied in paddy soil for rice growth, but its impact on As mobility and related bacteria remains poorly understood. In this study, a pot experiment was set up with two different types of sulfur treatments (element sulfur and Na₂SO₄) to evaluate the effect of sulfur fertilizers on As speciation in porewater, As fractions in soil, As accumulation in rice plants. Besides, rhizosphere bacterial composition and functional genes that might influence As mobility were also studied. The results revealed that the addition of 150 mg/kg Na₂SO₄ decreased As(III) and As(V) concentrations in soil porewater at maturation stage by 77% and 64%, respectively. With the same sulfur content, Na₂SO₄ was more effective than element sulfur. The addition of sulfur fertilizers promoted rice growth and reduced As accumulation in shoots, further reduced As translocation from root to above-ground parts by 39–59%. The addition of sulfur fertilizers had little effect on genes involved in As metabolism. However, the relative abundance of Fe(III) and sulfate reduction related genera increased with the addition of 150 mg/kg Na₂SO₄, consistent with the increase of Fe(III) reducing bacteria Geobacteraceae and sulfate reducing gene dsrA. The phenomenon likely influenced the decrease of As concentrations in soil porewater and rice uptake. The outcomes indicate that promoting Fe- and S- reducing bacteria in the rhizosphere by sulfur fertilizers may be one way to reduce As risk in the soil-rice system.

Arsenic (As) behavior in paddy soils couples with the redox process of iron (Fe) minerals. When soil is flooded, Fe oxides are transformed to soluble ferrous ions by accepting the electrons from Fe reducers. This process can significantly affect the fate of As in paddy fields. In this study, we show a novel technique to manipulate the Fe redox processes in paddy soils by deploying soil microbial fuel cells (sMFC). The results showed that the sMFC bioanode can significantly decrease the release of Fe and As into soil porewater. Iron and As contents around sMFC anode were 65.0% and 47.0% of the control respectively at day 50. The observed phenomenon would be explained by a competition for organic substrate between sMFC bioanode and the iron- and arsenic-reducing bacteria in the soils. In the vicinity of bioanode, organic matter removal efficiencies were 10.3% and 14.0% higher than the control for lost on ignition carbon and total organic carbon respectively. Sequencing of the 16S rRNA genes suggested that the influence of bioanodes on bulk soil bacterial community structure was minimal. Moreover, during the experiment a maximum current and power density of 0.31 mA and 12.0 mWm−2 were obtained, respectively. This study shows a novel way to limit the release of Fe and As in soils porewater and simultaneously generate electricity.

Rice growing in flooded paddy soil often accumulates considerable levels of inorganic and organic arsenic (As) species, which may cause toxicity to plants and/or pose a risk to human health. The bioavailability and toxicity of As in soil depends on its chemical species, which undergo multiple transformations driven primarily by soil microbes. However, the role of endophytes inside rice roots in As species transformation remains largely unknown. We quantified the abundances of microbial functional genes involved in As transformation in the endosphere and rhizosphere of rice roots growing in three paddy soils in a pot experiment. We also isolated 46 different bacterial endophytes and tested their abilities to transform various As species. The absolute abundances of the arsenate reductase gene arsC and the dissimilatory arsenate reductase gene arrA in the endosphere were comparable to those in the rhizosphere, whereas the absolute abundances of the arsenite methylation gene arsM and arsenite oxidation gene aioA in the endosphere were lower. After normalization based on the bacterial 16S rRNA gene, all four As transformation genes showed higher relative abundances in the endosphere than in the rhizosphere. Consistent with the functional gene data, all of the 30 aerobic endophytic isolates were able to reduce arsenate, but only 3 strains could oxidize arsenite. Among the 16 anaerobic endophytic isolates, 4 strains belonging to Desulfovibrio, Terrisporobacter or Clostridium could methylate arsenite and/or methylarsenite. Six strains of aerobic endophytes could demethylate methylarsenite, among which three strains also could reduce and demethylate methylarsenate. None of the isolates could demethylate dimethylarsenate. These results suggest that diverse endophytes living inside rice roots could participate in As species transformation and affect As accumulation and species distribution in rice plants.

Denitrifying anaerobic methane oxidation (DAMO) microorganisms, using nitrate/nitrite to oxidize methane, have been proved to be an important microbial methane sink in natural habitats. Increasing nitrogen deposit around the globe brings increased availability of substrates for these microorganisms. However, how elevated nitrogen level affects denitrifying methanotrophs has not been elucidated. In this study, sediment/soil samples from coastal wetland with continuous nitrogen input and paddy field with periodic nitrogen input were collected to investigate the influence of nitrogen input on the abundance and activity of denitrifying methanotrophs. The results indicated that nitrogen input significantly promoted DAMO microorganisms’ abundance and contribution to methane emission reduction. In the coastal wetland, the contribution rate of DAMO process to methane removal increased from 12.1% to 33.5% along with continuously elevated nitrogen level in the 3-year tracking study. In the paddy field, the DAMO process accounted for 71.9% of total methane removal when nitrogen fertilizer was applied during the growing season, exceeding the aerobic methane oxidation process. This work would help us better understand the microbial methane cycle and reduce uncertainties in the estimations of the global methane emission.

Biochar was prepared from rice straw and modified with 15% H₂O₂ and 1:1 HNO₃/H₂SO₄, respectively. The unmodified biochars and HCl treated biochars for carbonate removal were used as control. The biochars were added to the acid paddy soil collected from Langxi, Anhui Province, China at the rate of 30 g/kg. The paddy soil was flooded and then air-dried, and soil pH and Eh were measured in situ with pH electrode and platinum electrode during wet-dry alternation. Soil pH buffering capacity (pHBC) was determined by acid-base titration after the wet-dry treatment. Then, the simulated acidification experiments were carried out to study the changing trends of soil pH, base cations and exchangeable acidity. The results showed that soil pHBC was effectively increased and the resistance of the paddy soil to acidification was apparently enhanced with the incorporation of H₂O₂- and HNO₃/H₂SO₄-modified biochars. Surface functional groups on biochars were mainly responsible for enhanced soil resistance to acidification. During soil acidification, the protonation of organic anions generated by dissociation of these functional groups effectively retarded the decline of soil pH. The modification of HNO₃/H₂SO₄ led to greater increase in carboxyl functional groups on the biochars than H₂O₂ modification and thus HNO₃/H₂SO₄-modified biochars showed more enhancement in soil resistance to acidification than H₂O₂-modified biochars. After a wet-dry cycle, the pH of the paddy soil incorporated with HNO₃/H₂SO₄-modified biochar increased apparently. Consequently, the addition of HNO₃/H₂SO₄-modified biochar can be regarded as a new method to alleviate soil acidification. In short, the meaning of this paper is to provide a new method for the amelioration of acid paddy soils.

Biochar amendment has the potential to reduce methylmercury (MeHg) uptake by rice grains in soil-rice ecosystem. Considering that sulfur can strongly bind Hg and thus reduce its bioavailability, S-modified biochar has been used to immobilize Hg in soils. However, whether natural S-enriched biochar can further reduce Hg and MeHg phytoavailability remains unknown. Moreover, the rhizosphere is one of the most important microbial hotspots regulating the pollutant dynamics in terrestrial ecosystems. Therefore, it is of greater practical significance to examine the impact of biochar amendment on MeHg production and phytoavailability in the rhizosphere versus nonrhizosphere. Here, by conducting a pot experiment, we evaluated the efficacy of biochar derived from sulfur-enriched oilseed rape straw to reduce MeHg accumulation in rice. The results demonstrated that: (1) biochar-induced enhancement of chloride ion and sulfate levels in the overlying water and pore water facilitate microbial methylation of Hg and thus MeHg production in rhizosphere soil. (2) biochar amendment increased rhizosphere soil sulfur content and humic acid-like substances, strengthening MeHg binding to soil, and thus reducing grain MeHg levels by 47%–75%. Our results highlight the necessity to applying natural sulfur-rich biochar accompanied with exogenous sulfur to further reduce MeHg phytoavailability.