Rice straw is often returned directly to agricultural fields to recycle nutrients, improve soil quality, and avoid open burning. However, new research shows that this common practice may have complex and potentially undesirable effects on the accumulation of heavy metals in rice grains.
In a study published in Environmental and Biogeochemical Processes, researchers evaluated how six rice straw management strategies affected the accumulation of arsenic, cadmium, copper, nickel, lead, and zinc in rice. Their findings indicate that converting rice straw into biochar before applying it to soil may provide greater environmental and food safety benefits than directly incorporating untreated straw.
"Rice straw is a valuable agricultural resource, but its effects on contaminants cannot be judged by examining only one metal," said corresponding author Huan Zhong of Nanjing University. "Our results highlight the importance of evaluating multiple contaminants together and suggest that straw-derived biochar could help balance crop production, pollution control, and climate goals."
The researchers conducted a greenhouse pot experiment using cadmium-contaminated paddy soil collected from Jiangsu Province, China. They compared untreated soil with five straw management approaches, including direct straw incorporation, accelerated straw decomposition, soil pH adjustment, modified water management, and the application of rice straw-derived biochar.
Direct incorporation of rice straw produced sharply different effects among the six metals. Arsenic concentrations in rice grains increased by 73.1%, while copper and lead concentrations decreased by 13.8% and 89.3%, respectively. Cadmium, nickel, and zinc concentrations did not change significantly under the experimental conditions.
Attempts to reduce the unintended increase in metal accumulation by accelerating straw decomposition, raising soil pH, or changing water availability were not consistently successful. In some cases, these measures created additional problems. Water-saving management, for example, caused grain cadmium concentrations to rise approximately 30-fold and exceed China's national food safety limit.
These findings demonstrate why recommendations based on a single contaminant can be misleading. Organic matter released during straw decomposition may bind strongly to some metals, such as copper and lead, while changing soil chemistry and microbial activity in ways that increase the mobility or transformation of other elements, including arsenic.
By comparison, rice straw-derived biochar applied at a relatively low rate of approximately 0.3% did not significantly increase any of the six metals in rice grains. The biochar treatment also reduced copper and lead accumulation, improved several soil properties, and produced the highest grain and whole-plant biomass among the tested treatments.
Biochar is produced by heating biomass under oxygen-limited conditions. This process creates a stable, carbon-rich material that can be added to soil. Converting straw into biochar may also help avoid air pollution from open burning and reduce the greenhouse gas emissions associated with the decomposition of untreated straw in flooded paddies.
The authors caution that the experiment was conducted in pots using one type of contaminated soil. Field studies across different soils, climates, and rice varieties will be needed before large-scale recommendations can be made. Economic factors, including straw collection, biochar production, transportation, and application costs, must also be considered.
The study nevertheless identifies low-dose straw-derived biochar as a promising strategy for managing agricultural residues while supporting rice production, reducing heavy metal exposure, and improving environmental sustainability.
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Journal reference: Liao J, Ning W, Gong Y, Tang W, Zhong H. 2026. Incorporating rice straw in the form of biochar: a sustainable measure to protect humans from heavy metal exposure. Environmental and Biogeochemical Processes 2: e012 doi: 10.48130/ebp-0026-0007
https://www.maxapress.com/article/doi/10.48130/ebp-0026-0007
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About the Journal:
Environmental and Biogeochemical Processes (e-ISSN 3070-1708) is a multidisciplinary platform for communicating advances in fundamental and applied research on the interactions and processes involving the cycling of elements and compounds between the biological, geological, and chemical components of the environment.
