Rice straw is commonly returned to agricultural soils to recycle nutrients, improve soil health, and avoid open-field burning. However, organic matter released during decomposition can alter soil acidity, redox conditions, microbial activity, and metal mobility. Earlier studies have generally examined individual contaminants and produced inconsistent findings: straw incorporation may reduce cadmium uptake but promote the accumulation of arsenic or mercury in rice. Because several heavy metals often coexist in paddy soils, recommendations based on a single contaminant may overlook important trade-offs. Furthermore, the effectiveness of measures such as accelerated decomposition, soil pH adjustment, and water management in controlling these risks remains uncertain, creating a need for an assessment of multiple metals under different straw-management scenarios.
A study (DOI: 10.48130/ebp-0026-0007) published in Environmental and Biogeochemical Processes on 30 June 2026 by Wenli Tang's & Huan Zhong's team, Nanjing University, reports that biochar can preserve the agricultural benefits of rice straw while avoiding the overall increase in grain heavy-metal accumulation associated with its direct incorporation.
The researchers conducted a greenhouse pot experiment using cadmium-contaminated paddy soil and established six management treatments, each with three replicates. These included a control without straw; direct rice-straw incorporation; straw combined with a decomposition agent; decomposer-treated straw plus calcium carbonate to raise soil pH; decomposer-treated straw combined with reduced water supply after flowering; and biochar produced by heating rice straw at 600 °C for two hours under oxygen-limited conditions. Untreated straw was applied at 1% of soil weight, whereas mass loss during pyrolysis resulted in a biochar application rate of approximately 0.3%. Rice plants were grown for 131–135 days. Throughout the experiment, the team measured soil pH, dissolved and total soil organic carbon, redox potential, cadmium availability, plant height, biomass, and grain yield. At harvest, arsenic, cadmium, copper, nickel, lead, and zinc in grains and other plant tissues were quantified using inductively coupled plasma mass spectrometry. The researchers also reviewed published studies to compare their experimental findings with previously reported effects of straw incorporation. Direct straw incorporation increased dissolved organic carbon and generally promoted plant growth, but it produced strongly metal-specific effects. Grain arsenic rose by 73.1%, whereas copper and lead declined by 13.8% and 89.3%, respectively; cadmium, nickel, and zinc showed no significant change. Neither faster straw decomposition, higher soil pH, nor modified water management reliably eliminated the increase in metal accumulation. Water-saving management was particularly problematic: grain cadmium increased 30-fold and reached 15 times the applicable national food-safety limit. By contrast, straw-derived biochar did not significantly increase any measured metal in the grain and significantly reduced copper and lead. It also improved soil properties, supported plant growth, and temporarily lowered extractable cadmium. A literature dataset containing 56 observations reinforced the need for multi-metal evaluation, showing average increases of 30.3% for cadmium and 28.5% for arsenic following direct straw incorporation, although responses varied with soil conditions.
Overall, the study questions whether returning untreated rice straw is always beneficial. Although direct incorporation is convenient and may promote plant growth, it can alter contaminant mobility and increase dietary exposure to certain heavy metals. Converting straw into biochar offers a more balanced approach, recycling agricultural biomass without raising metal concentrations in rice grain. However, field-scale trials, contaminant-speciation analyses, and economic assessments remain necessary before widespread adoption because biochar production requires straw collection, processing infrastructure, extra labor, investment, and resources.
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References
DOI
10.48130/ebp-0026-0007
Original Source URL
https://doi.org/10.48130/ebp-0026-0007
Funding information
We appreciate the financial support from the Huanghuai Lab Sci-Tech Innovation Project (240700002), the Natural Science Foundation of Jiangsu Province (BK20230082), and the National Natural Science Foundation of China (42107223).
About Environmental and Biogeochemical Processes
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.