A research team developed oxygen-nanobubble-loaded biochar (ONBC) that releases oxygen into flooded rice rhizospheres, maintaining oxidizing conditions and limiting cadmium (Cd) uptake. Unlike conventional biochar, ONBC converts mobile Cd into more stable soil fractions, supports beneficial microbial functions, and improves plant growth. Greenhouse experiments showed lower Cd concentrations in rice roots and shoots. This resource-efficient material could help remediate Cd-contaminated paddies and support safer rice production, although field trials must confirm its long-term effectiveness, scalability, economic feasibility, and sustainability.
Cadmium contamination threatens agricultural ecosystems, crop quality, and food safety, with rice particularly vulnerable because flooding changes soil redox conditions and may increase Cd mobility. Biochar is widely investigated as a soil amendment because its porous structure, alkalinity, surface functional groups, and ion-exchange capacity can retain heavy metals. However, ordinary biochar generally acts as a passive sorbent. Oxygen is rapidly depleted in flooded soils, creating reducing conditions that can dissolve iron and manganese phases and release previously immobilized Cd. Iron-loaded biochar can supply additional iron, but without sustained oxygenation, its benefits may be temporary. A material capable of simultaneously regulating rhizosphere oxygen, metal speciation, and microbial activity is therefore needed.
A study (DOI: 10.48130/bchax-0026-0015 ) published in Biochar X on 09 June 2026 by Zhimin Sha's team, Shanghai Jiao Tong University, reports that ONBC maintained an oxidizing rhizosphere and shifted Cd from mobile pools into more stable forms, thereby reducing its accumulation in rice tissues.
The researchers produced biochar from corn straw pyrolyzed at 500 °C and prepared ONBC through repeated vacuum extraction and high-purity oxygen pressurization, achieving an oxygen-loading capacity of 15.6 mg O₂ per gram of biochar. They then conducted a greenhouse experiment using Cd-contaminated paddy soil and rice ( Oryza sativa L. cv. Qingjiao 307). Four treatments—with no amendment, pristine biochar, iron-loaded biochar, or ONBC—were tested at four replicates each. The amended materials were applied at 1% by soil weight, and a water layer was maintained throughout cultivation. Researchers monitored dissolved oxygen, soil pH, and rhizosphere redox potential over 90 days. They also measured rice growth, root physiology, iron plaque formation, Cd concentrations, and the distribution of Cd among five soil fractions. Metagenomic sequencing was used to characterize bacterial diversity and functional genes, while structural equation modeling identified the main pathways controlling plant Cd accumulation. ONBC maintained dissolved oxygen between 2.20 and 4.23 mg L⁻¹ during cultivation and produced an average redox potential of 242.3 mV, substantially higher than those of the other treatments. After 90 days, its dissolved oxygen level was approximately 4.9 times that of the unamended soil. Rather than relying mainly on alkalization, ONBC promoted oxidative iron and manganese cycling. Exchangeable Cd fell by about 67%–70% relative to the control, while carbonate-bound Cd declined by approximately 46%. Together, labile Cd fractions decreased from roughly 28%–30% of extracted Cd in the other treatments to about 13% with ONBC. Stable fractions associated with iron and manganese oxides, organic matter, and residual minerals consequently increased to approximately 87%. Root and shoot Cd concentrations were respectively 2.7 and 1.9 times lower than in the control. ONBC also increased bacterial diversity, enriched iron- and manganese-oxidizing microorganisms, strengthened microbial Cd export and detoxification functions, and stimulated carbon, nitrogen, and phosphorus turnover. Structural equation modeling indicated that changes in bulk-soil Cd speciation, driven by redox regulation and microbial restructuring, contributed more to reduced plant Cd than iron plaque on root surfaces.
Overall, the study shows that oxygen-nanobubble-loaded biochar transforms crop residues into an active platform that regulates the rice rhizosphere. By sustaining oxygen, stabilizing cadmium, reshaping microbial functions, and promoting plant growth, ONBC offers a promising remediation strategy for contaminated paddies. However, greenhouse testing covered only one soil, rice cultivar, and application rate. Multi-site field trials should therefore assess oxygen-release durability, grain cadmium concentrations, crop yields, application costs, and long-term environmental performance before large-scale agricultural use can be safely recommended widely.
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References
DOI
Original Source URL
https://doi.org/10.48130/bchax-0026-0015
Funding information
The authors would like to acknowledge funding from the Shanghai Agricultural Applied Technology Development Program, China (A2025001), and the 2025 Shanghai Municipal Grassroots Science Popularization Action Plan Project (JCKP2025-29).
About Biochar X
Biochar X (e-ISSN 3070-1686) is an open access, online-only journal aims to transcend traditional disciplinary boundaries by providing a multidisciplinary platform for the exchange of cutting-edge research in both fundamental and applied aspects of biochar. The journal is dedicated to supporting the global biochar research community by offering an innovative, efficient, and professional outlet for sharing new findings and perspectives. Its core focus lies in the discovery of novel insights and the development of emerging applications in the rapidly growing field of biochar science.
Biochar X
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An oxygen-nanobubble-loaded biochar for cadmium stabilization in contaminated paddy soil
9-Jun-2026
The authors declare that they have no competing interests.