Keeping a paddy’s root zone oxidising, rather than simply adsorbing the metal, shifted cadmium into stable mineral forms and left 2.7 times less in rice roots.

Rice has a cadmium problem that follows directly from how it is grown. Flooding a paddy starves the soil of oxygen, and in those reducing conditions cadmium becomes mobile and available to roots — which is why rice is the main dietary source of cadmium for much of the world’s population, and why the metal accumulates in grain grown on contaminated land.

Biochar is a standard remedy. Charred biomass added to soil adsorbs metals onto its surfaces and holds them there, and it works, up to a point. A study in Biochar X proposes doing something more ambitious with it: loading the biochar with oxygen nanobubbles so that it does not merely bind cadmium but changes the chemistry that makes cadmium mobile in the first place.

The corresponding author is Zhimin Sha of Shanghai Jiao Tong University.

An oxygen reservoir in the root zone

The material is an oxygen-nanobubble-loaded biochar, abbreviated ONBC, and in a greenhouse pot experiment it was compared against untreated biochar, iron-loaded biochar and unamended soil.

The first result is that the oxygen stayed put long enough to matter. ONBC released oxygen gradually across flooded cultivation, holding dissolved oxygen at roughly 3 to 4 milligrams per litre through the key growth stages, and keeping rhizosphere redox potential positive. The untreated soil and the other biochar treatments trended toward oxygen depletion, as flooded soils do.

Nanobubbles are the reason that is possible. Gas bubbles below about a micrometre across resist rising and bursting, which is what lets an oxygen charge persist in waterlogged soil for weeks rather than escaping in hours — and the biochar’s pore structure provides somewhere for them to sit.

Where the cadmium went

The chemistry follows from the redox state. The most mobile exchangeable cadmium fraction fell by about 67 to 70 per cent compared with untreated control, and the metal shifted toward forms bound to iron and manganese minerals, to organic matter, and into residual mineral phases.

That redistribution is the mechanism rather than a side effect. Under oxidising conditions, dissolved iron and manganese precipitate as oxides, and those oxide surfaces are efficient cadmium sinks; the metal is not removed from the soil but relocated into compartments that roots cannot reach. Keeping the soil oxidising recruits the soil’s own iron chemistry to do the sequestration.

In the plants, rice roots contained 2.7 times less cadmium than roots in untreated soil, and shoots 1.9 times less. ONBC outperformed both conventional biochar and the iron-loaded version.

“Our results show that oxygen loading can transform biochar from a passive adsorbent into an active platform that regulates soil chemistry and microbial activity around plant roots,” Sha said.

The microbial half

The second half of that claim was tested by sequencing. Metagenomic analysis found that ONBC increased bacterial diversity and enriched microbes associated with iron and manganese oxidation — the organisms that generate the oxide surfaces the cadmium ends up on.

The functional gene profile shifted in a consistent direction. Genes associated with cadmium entering microbial cells were reduced, while genes for cadmium export, detoxification, stress response and binding increased, and genes for carbon, nitrogen and phosphorus turnover were also enhanced.

This is the part that distinguishes the approach from adsorption. A soil amendment that changes which microbes thrive, and what those microbes do with a metal, is intervening in a system that continues operating after the amendment’s own capacity is saturated.

The plants were measurably better off as well: compared with untreated control, ONBC increased whole-plant fresh biomass by 44.7 per cent, root activity by 64.5 per cent and superoxide dismutase activity by 85.4 per cent. The last of those is an antioxidant enzyme, and a rise in it is a conventional marker of reduced oxidative stress.

“This combined redox and biological regulation offers a promising new direction for reducing cadmium risks in flooded agricultural soils,” Sha said.

The measurement that is missing

The authors are direct about the limitation that matters most, and it deserves to be stated as plainly as they state it: grain cadmium could not be reliably measured, because the plants had not reached full grain maturity.

Grain cadmium is the entire public health endpoint. Everything else — the exchangeable fraction, the root and shoot concentrations, the microbial gene profile — is an intermediate on the path to the number that determines whether rice from a contaminated field is safe to eat. Root concentration and grain concentration are correlated but not interchangeable, since rice has internal transport and sequestration steps between root and grain that vary by variety.

So the honest summary is that the soil chemistry works, the plant uptake is substantially reduced at two points in the plant, and the consequence for the edible part has not been measured.

Greenhouse pots

The authors also note that this was a controlled greenhouse pot experiment, and that field trials across different soils, climates and rice varieties are needed before broad use.

Pots are a particularly consequential simplification for this question. A pot has no water table, no lateral flow, and a volume of soil small enough for an amendment to influence all of it — whereas a paddy field is a hydrological system with inflow and drainage, and the oxygen a nanobubble biochar releases has somewhere to go. Whether dissolved oxygen can be held at 3 to 4 milligrams per litre in a real field, at an application rate anyone would pay for, is the question the pot cannot answer.

Cost is unaddressed too. Manufacturing biochar and then charging it with nanobubbles is more expensive than manufacturing biochar, and cadmium remediation competes against the cheaper existing options of water management, liming and switching to low-accumulating cultivars.

What the study establishes is a mechanism worth testing at scale, with unusually complete evidence for it — soil chemistry, metal speciation, plant uptake, microbial community and gene function all pointing the same way. The paper was published on 9 June 2026. The authors declare no competing interests; the release does not state funding sources.

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Notes

1. “An oxygen-nanobubble-loaded biochar for cadmium stabilization in contaminated paddy soil.” Biochar X, volume 2, article e018, 2026. DOI: 10.48130/bchax-0026-0015.
2. Shenyang Agricultural University Collaborative Journals. “Oxygen nanobubbles turn biochar into an active defense against cadmium in flooded rice soils.” EurekAlert!, 4 August 2026. https://www.eurekalert.org/news-releases/1138826.

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