Rice feeds billions of people, but flooded paddy fields are also an important source of methane, a greenhouse gas far more powerful than carbon dioxide over a 100-year period. A new five-year field study suggests that the way biochar is applied may determine whether rice farming can achieve durable climate benefits.
In a study published in Biochar, researchers tested whether a single large dose of biochar or smaller annual applications could better reduce methane emissions from paddy fields under two water management systems. The team found that continuous biochar amendment combined with water-saving irrigation delivered the strongest long-term methane mitigation, while a one-time application lost much of its effect over time.
"Biochar has often been promoted as a promising tool for reducing methane emissions from rice fields, but most studies only capture short-term responses," said corresponding author Zhongxue Zhang. "Our five-year experiment shows that long-term performance depends strongly on both water management and the biochar application strategy."
The researchers conducted the field experiment from 2018 to 2022 in Heilongjiang Province, China. They compared six treatments: flooded irrigation without biochar, flooded irrigation with one biochar application, flooded irrigation with annual biochar application, controlled water-saving irrigation without biochar, controlled irrigation with one biochar application, and controlled irrigation with annual biochar application. The one-time treatment used 12.5 tons of biochar per hectare in the first year, while the continuous treatment used 2.5 tons per hectare each year.
In the first year, the single large biochar application was highly effective, reducing cumulative methane emissions by 18.87% to 36.32% compared with other treatments. However, its mitigation effect gradually weakened over the five-year period, especially under water-saving irrigation. The authors suggest that repeated wet and dry cycles may accelerate biochar aging, reducing its ability to improve soil conditions and suppress methane production.
By contrast, annual biochar amendment maintained and strengthened methane mitigation over time. Under water-saving irrigation, continuous biochar amendment reduced five-year cumulative methane emissions by 29.32% compared with no biochar and by 18.26% compared with one-time biochar application. This treatment also achieved the lowest methane-attributed greenhouse gas intensity and the lowest net greenhouse gas emissions across the study period.
"Our results indicate that a one-time application may look promising in the short term, but it may not provide sustained mitigation under water-saving irrigation," said corresponding author Xiaoyuan Yan. "Continuous biochar amendment can replenish active surfaces and help maintain soil conditions that limit methane production and favor methane oxidation."
The study also examined the mechanisms behind the emissions changes. Using random forest analysis and structural equation modeling, the researchers identified soil redox potential, dissolved organic carbon, and ammonium nitrogen as key soil factors controlling methane emissions. The best-performing treatment maintained higher soil redox potential and ammonium nitrogen, lower dissolved organic carbon, the lowest methane production potential, and the highest methane oxidation potential.
Importantly, the climate benefits did not come at the expense of food production. Continuous biochar amendment under water-saving irrigation maintained the highest average rice yield among the tested treatments, suggesting a possible pathway for climate-resilient rice cultivation.
"These findings highlight the need for multi-year field studies," Zhang said. "Short experiments may overestimate the long-term value of one-time biochar application. For farmers and policymakers seeking durable methane mitigation, continuous biochar use with water-saving irrigation may be a more reliable strategy."
The authors conclude that integrating annual biochar amendment with water-saving irrigation could help rice systems reduce methane emissions, improve soil carbon storage, and sustain crop productivity, supporting both climate mitigation and food security goals.
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Journal Reference: Han, Y., Chen, P., Zhang, Z. et al. Continuous biochar amendment to achieve long-term CH4 mitigation in paddy fields under water-saving irrigation: a 5-year experiment. Biochar 8, 70 (2026).
https://doi.org/10.1007/s42773-026-00578-z
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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.
