Biochar has long been promoted as a promising tool to reduce greenhouse gas emissions from agricultural soils. However, new research suggests that its climate benefits may not be as stable as once believed.
A study published in Biochar reveals that biochar can both decrease and increase emissions of nitrous oxide, a potent greenhouse gas, depending on how long it has been in the soil. The findings highlight the importance of considering long-term impacts when evaluating climate solutions.
Nitrous oxide is a major contributor to global warming and ozone depletion, and agricultural soils are its largest human-related source. Acidic soils, which are common in many farming regions, are particularly prone to releasing this gas. Scientists have widely explored biochar, a carbon-rich material produced from biomass, as a way to mitigate these emissions.
In the new study, researchers examined soils that had received biochar at different times, ranging from three to nine years earlier. By combining laboratory experiments, isotope tracing, and microbial analyses, the team uncovered how biochar influences nitrogen cycling processes that control nitrous oxide emissions.
The results showed a clear contrast over time.
In the short term, biochar reduced nitrous oxide emissions by up to 84 percent. This reduction occurred because biochar suppressed the production of nitrous oxide while simultaneously enhancing its conversion into harmless nitrogen gas. The process was linked to beneficial changes in soil microbes, particularly those carrying the nosZ gene, which enables microbes to consume nitrous oxide.
"In the early years after application, biochar creates conditions that favor microbes capable of removing nitrous oxide from the soil," said the study's corresponding author. "This leads to a strong mitigation effect."
However, the long-term results told a different story.
After nine years, biochar-treated soils emitted significantly more nitrous oxide than untreated soils. The researchers found that although biochar continued to suppress the production of nitrous oxide, it more strongly inhibited the microbial processes responsible for reducing it. As a result, nitrous oxide accumulated and was released into the atmosphere.
This shift was associated with declines in key microbial groups and reduced availability of dissolved organic carbon, an essential energy source for denitrifying microbes. At the same time, fungal pathways that produce nitrous oxide became more dominant. Unlike certain bacteria, these fungi lack the ability to convert nitrous oxide into nitrogen gas, leading to greater emissions.
The study demonstrates that biochar's effects are not static but evolve over time as the material ages and interacts with soil environments.
"Our findings show that the climate benefits of biochar are time dependent," the authors noted. "What works in the short term may not hold in the long term."
The researchers emphasize that biochar still holds potential as a climate mitigation strategy, but its application must be carefully managed. Long-term monitoring, soil-specific strategies, and a deeper understanding of microbial processes will be essential to ensure that biochar delivers sustained environmental benefits.
These insights provide a more nuanced view of biochar's role in sustainable agriculture and highlight the need to evaluate climate solutions across longer time scales.
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Journal Reference: Guo, S., Lin, H., Li, Z. et al. Divergent legacy effects of biochar on nitrous oxide emissions in acidic soils driven by altered microbial N pathways. Biochar 8, 40 (2026).
https://doi.org/10.1007/s42773-025-00558-9
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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.
