Rice is a daily staple for billions of people, but in some agricultural regions, the soil itself poses a hidden threat. Cadmium, a toxic heavy metal naturally present in certain geological formations, can accumulate in rice grains and enter the human food chain. At the same time, selenium, an essential nutrient for human health, may remain trapped in the soil and unavailable to crops.
Now, researchers have discovered that specially designed biochar-based soil amendments can address both challenges simultaneously, reducing cadmium accumulation in rice by up to 85.4% while increasing selenium levels by up to 357.5%.
The findings, published in the journal Biochar, demonstrate a practical strategy for improving both food safety and nutritional quality in rice-producing regions affected by naturally elevated metal concentrations.
"Our findings suggest that soil management can do more than simply reduce harmful contaminants. By carefully regulating soil chemistry, we can also improve the nutritional value of crops, creating opportunities for safer and healthier food production," said corresponding author Professor Deyi Hou of Tsinghua University.
Tackling two challenges with one solution
The research team conducted field experiments in a karst region of Southwest China, where limestone-derived soils naturally contain elevated concentrations of cadmium and selenium.
Unlike pollution caused by industrial activities, these naturally occurring elements cannot be easily eliminated by controlling external contamination sources.
To address this challenge, researchers developed three biochar-based amendments containing clay minerals, cow manure, or zero-valent iron. Biochar is a carbon-rich material produced by heating biomass under limited oxygen conditions.
The amendments were applied to rice paddies at different rates, and their effects were monitored across two consecutive growing seasons.
All three treatments successfully reduced cadmium accumulation while increasing selenium concentrations in rice grains. The amendments also improved rice yields, with reported increases ranging from 29.7% to 69.2%.
Among the treatments, iron-enriched biochar delivered the fastest and most consistent results, reducing grain cadmium below China's national safety threshold of 0.2 mg/kg during both growing seasons at application rates of 10 to 20 tonnes per hectare.
Clay-based and manure-based amendments achieved comparable cadmium control during the second growing season, suggesting that their effectiveness improves as they interact with soil over time.
How biochar changes soil chemistry
The researchers found that biochar treatments increased soil pH, shifting acidic soils toward more neutral conditions.
This change helped immobilize cadmium, making it less available for plant uptake, while simultaneously increasing selenium mobility and availability.
The key breakthrough lies in controlling two elements with contrasting chemical behaviors through a single soil treatment.
Importantly, the approach utilized selenium already present in the soil, without requiring additional selenium fertilizers.
A promising path toward sustainable agriculture
The findings also highlight practical trade-offs. Iron-enriched biochar offers rapid remediation but costs more, while clay-based and manure-based alternatives provide potentially more affordable options for large-scale agricultural use.
Although further research is needed to evaluate long-term performance across different soil types, the study provides valuable evidence that biochar-based amendments can support safer crop production in geologically challenging environments.
By turning agricultural residues and other accessible materials into functional soil amendments, this strategy could help protect food supplies, improve crop nutrition, and support more sustainable farming practices.
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Journal Reference: Zhou, P., Xu, Y., Guo, J. et al. Field application of biochar-based amendments for safe crop production in karst paddy with a high geological background: Unraveling Cd-Se dynamics. Biochar 8, 143 (2026).
https://doi.org/10.1007/s42773-026-00657-1
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About Biochar
Biochar (e-ISSN: 2524-7867) is the first journal dedicated exclusively to biochar research, spanning agronomy, environmental science, and materials science. It publishes original studies on biochar production, processing, and applications—such as bioenergy, environmental remediation, soil enhancement, climate mitigation, water treatment, and sustainability analysis. The journal serves as an innovative and professional platform for global researchers to share advances in this rapidly expanding field.
