Modified Biochar Pairs With Water Management to Curb Toxic Metals in Rice

A greenhouse investigation conducted by agricultural scientists has demonstrated that pairing iron-manganese modified biochar with targeted irrigation regimes resolves a persistent agronomic dilemma in paddy soils co-contaminated with cadmium and mercury. Cultivating rice in soils tainted by both toxic heavy metals has historically presented severe ecological challenges because cadmium and mercury exhibit opposing geochemical behaviors when soil water levels fluctuate. Flooded paddies generally trigger chemical reduction that immobilizes cadmium but simultaneously accelerates the microbial methylation of mercury into highly neurotoxic methylmercury. In a study published in Biochar, researchers evaluated an integrated soil management system combining engineered corn straw biochar amended with iron and manganese oxides alongside continuous flooding or continuous aerobic irrigation to determine whether tailored soil amendments can neutralize this redox trade-off and protect grain safety.

The experimental work utilized hydragric paddy topsoil developed from yellow earth gathered from the upper twenty centimeters of an agricultural field in Tongren City within Guizhou Province, China. Prior analysis confirmed baseline contamination levels of 0.76 milligrams of cadmium per kilogram of soil and 5.4 milligrams of mercury per kilogram. Soil treatments evaluated unamended control soil, unmodified corn straw biochar, and iron-manganese modified biochar, all tested under either continuous flooding with three centimeters of standing water or continuous aerobic watering regulated at eighty percent of soil water-holding capacity. Basal fertilizers were applied consistently to all pots, with supplementary feeding introduced at heading to monitor metal translocation and plant development throughout the life cycle.

Monitoring soil chemical properties over the growing season revealed that water management decisively drove soil redox potential, while biochar additions shifted soil reaction and carbon storage. Soil redox potential ranged from minus 109.42 to positive 278.60 millivolts under flooded irrigation, compared to a range between minus 70.76 and positive 557.67 millivolts in aerobic pots. Flooding induced oxygen depletion, driving anaerobic microbial decomposition that generated substantial pools of reducing substances and lowered the redox potential while raising soil solution pH by 0.03 to 0.09 units relative to aerobic conditions at harvest. Unmodified biochar and iron-manganese modified biochar further increased soil pH due to their inherent alkalinity.

These sharp geochemical shifts exerted diametrically opposing effects on initial metal extractability in the rhizosphere. In unamended soils, continuous flooding reduced plant-available cadmium extracted by diethylenetriaminepentaacetic acid by 3.47 percent compared to aerobic conditions, because reducing conditions stimulate sulfate-reducing bacteria that convert sulfate ions to sulfide, prompting the precipitation of insoluble cadmium sulfide. Conversely, aerobic soils promoted sulfide oxidation, thereby releasing mobile cadmium ions. Mercury demonstrated the reverse pattern: continuous aerobic irrigation reduced bioavailable mercury extracted by sodium thiosulfate by 20.66 to 22.29 percent compared to continuous flooding, where reducing conditions solubilized inorganic mercury and quadrupled soil solution methylmercury concentrations. The introduction of iron-manganese modified biochar suppressed both extractable metal pools across all watering regimes, cutting available cadmium by 1.81 percent under flooding and 3.41 percent under aerobic conditions, while lowering bioavailable mercury by 27.84 percent and 26.86 percent, respectively, by shifting exchangeable and carbonate-bound fractions into iron-manganese oxide, organic-bound, and residual fractions.

Root surface biogeochemistry played a primary role in preventing cadmium from translocating into vegetative tissues and edible grains. Flooding significantly enhanced the formation of iron-manganese plaques on fresh rice roots, yielding dithionite-citrate-bicarbonate extractable iron concentrations 6.49 to 12.05 times higher and manganese levels 2.81 to 5.34 times higher than aerobic treatments. When plants adapt to flooded hypoxia, root aerenchyma conduits transport oxygen downward, venting gas into the rhizosphere where ferrous and manganous ions oxidize and precipitate as an iron-manganese plaque sheath. The iron-manganese modified biochar supplied additional soluble iron and manganese, amplifying plaque development. Under flooded conditions, biochar amendment enhanced plaque iron by 10.85 percent and elevated plaque manganese 8.33-fold over unamended controls. This mineral coating functioned as an adsorption barrier, intercepting cadmium ions before symplastic or apoplastic entry into root epidermal cells.

These combined interfacial barriers produced substantial differences in toxic metal accumulation across rice tissues at final harvest. Cadmium concentrations in unamended rice grains grown under aerobic irrigation were 4.00 to 8.49 times higher than those grown under continuous flooding. However, applying iron-manganese modified biochar under continuous flooding slashed grain cadmium concentration by 72.62 percent to 0.05 milligrams per kilogram, far below the national food safety limit of 0.2 milligrams per kilogram established under Chinese standard GB 2762-2017. Two-way analysis of variance confirmed a statistically significant interaction between water management and biochar amendment on grain cadmium. The amendment also reduced cadmium in roots by 56.08 percent, in stems by 72.65 percent, and in leaves by 68.30 percent under flooding. In aerobic pots, the biochar mitigated the aeration-induced surge in cadmium, reducing grain accumulation by 41.92 percent relative to unamended aerobic controls.

For mercury, tissue accumulation patterns flipped, yet iron-manganese modified biochar achieved comparable remediation efficacy. Grains harvested from unamended flooded treatments contained total mercury concentrations between 0.05 and 0.10 milligrams per kilogram, which was 2.55 to 3.46 times higher than aerobic levels of 0.02 to 0.03 milligrams per kilogram. Methylmercury accounted for 64.82 to 69.85 percent of total grain mercury under flooding, but dropped to 32.68 to 33.95 percent under aerobic management. Coupling aerobic irrigation with iron-manganese modified biochar produced the lowest contamination values, yielding 0.02 milligrams per kilogram of total mercury and 6.89 micrograms per kilogram of methylmercury. Biochar functional groups, including surface oxygen complexes and metal-oxygen bonds, chemically stabilized inorganic mercury substrates. This restricted the pool of mercury accessible for biological uptake and suppressed total grain mercury accumulation by 32.04 percent and methylmercury by 38.83 percent relative to unamended aerobic controls.

High-throughput Illumina sequencing of the bacterial V4-V5 hypervariable region yielded 733,837 quality-filtered reads across the 18 soil samples, revealing how environmental regimes structured microbial communities. PCoA ordination separated samples into two distinct clusters governed primarily by water management, with the first two coordinates accounting for 72.32 percent and 8.33 percent of overall taxonomic variance. Flooding elevated community richness indices, including ACE and Chao1, while reducing Shannon diversity due to oxygen exclusion. Firmicutes dominated flooded soils at relative abundances of 44.65 to 47.77 percent, whereas aerobic irrigation enriched Actinobacteria and Proteobacteria. At the family level, flooding promoted Clostridiaceae and Geobacteraceae, both acknowledged drivers of anaerobic mercury methylation. Biochar application expanded alpha diversity across all treatments by enhancing soil porosity and aeration, moderating the extreme community imbalances triggered by prolonged water stagnation.

Profiling specific bacterial genera carrying the hgcA gene cluster revealed the microbiological mechanism underpinning mercury methylation control. Total hgcA-carrying methylators accounted for 2.34 to 2.58 percent of communities under continuous flooding, but only 1.51 to 1.78 percent under aerobic irrigation, with dominant genera comprising Defluviicoccus, Geobacter, Anaerolinea, Desulfitobacterium, Syntrophus, and Syntrophorhabdus. Although iron-manganese modified biochar marginally reduced soil redox potential, its chemical immobilization of mercury substrates paradoxically suppressed total hgcA-carrying methylators by 0.22 percent under flooding and 0.26 percent under aerobic regimes, notably decreasing Geobacter, Desulfovibrio, and Desulfosporosinus. Network analysis demonstrated that biochar treatments enhanced microbial co-occurrence complexity, modularity, and average degree connectivity. In aerobic biochar soils, metal-resistant Planctomycetota emerged as a keystone taxon, while flooded biochar systems enriched Deinococcus, Herbaspirillum, and Novosphingobium, functional groups capable of producing heavy-metal-chelating extracellular polymeric substances and siderophores that buffer rhizosphere toxicity.

To integrate these multidimensional interactions, the investigators applied partial least squares path modeling, which revealed that water management exerted a stronger overall influence on grain metal accumulation than amendment addition alone. Goodness-of-fit evaluations reached 0.6788 for cadmium models and 0.6864 for mercury models, confirming that available soil metals and root surface plaque formation directly controlled final grain loading. Health risk modeling indicated that combining continuous flooding with iron-manganese modified biochar lowered the target hazard quotient for cadmium from 1.23 to 0.31, eliminating non-carcinogenic exposure risks for rice consumers. Conversely, combining aerobic irrigation with the modified biochar lowered target hazard quotients for total mercury and methylmercury to 0.22 and 0.41, respectively. Despite these promising greenhouse outcomes, the authors noted critical limitations, including the reliance on small-scale pot trials that do not reflect complex field hydrology, unpredictable weather disruptions, long-term biochar weathering, and the economic hurdles of industrial-scale amendment manufacturing, underscoring the necessity of multi-year field evaluations before broad agronomic deployment.

Subject of Research: Remediation of cadmium and mercury co-contaminated paddy soil using modified biochar and water management

Article Title: Synergistic effects of Fe–Mn modified biochar and water management on remediation of Cd and Hg co-contaminated soils

Article References: Sun, T., Yang, W., Sun, Y., Wang, L., & Liang, X. (2026). Synergistic effects of Fe–Mn modified biochar and water management on remediation of Cd and Hg co-contaminated soils. Biochar, 8(1), Article 121. https://doi.org/10.1007/s42773-026-00616-w

Image Credits: AI Generated

DOI: 10.1007/s42773-026-00616-w

Keywords: Biochar, Cadmium Remediation, Mercury Methylation, Paddy Soil, Water Management, Food Safety, Soil Microbiology, Heavy Metals, Synergistic, effects, modified, water