Not all biochars cut greenhouse gas emissions, study finds
image: Key biochar properties linked to denitrification products in a calcareous soil
Credit: María L. Cayuela, Oliver Spott, María B. Pascual, María Sánchez-García & Miguel A. Sánchez-Monedero
Biochar is often promoted as a promising tool for climate-smart agriculture, thanks to its ability to store carbon in soils and, in many cases, reduce emissions of nitrous oxide, a powerful greenhouse gas. But a study published in Biochar shows that the climate benefit of biochar is not guaranteed. The way biochar is produced, and the chemical groups present on its surface, can determine whether it helps reduce nitrous oxide or unexpectedly increases it.
Nitrous oxide, or N2O, is a long-lived greenhouse gas mainly released from fertilized agricultural soils. One important source is denitrification, a microbial process in which nitrate is transformed into gaseous nitrogen compounds, including N2O and harmless dinitrogen gas, N2.
In the new study, researchers María L. Cayuela, Oliver Spott, María B. Pascual, María Sánchez-García and Miguel A. Sánchez-Monedero investigated how different biochars influence denitrification in a calcareous agricultural soil. The team tested 22 well-characterized biochars made from common Mediterranean plant residues, including pruning waste, rice straw, tomato crop residues, grape stalks and olive mill waste. The biochars were produced at either 400 °C or 600 °C.
Most of the biochars reduced N2O emissions, but several had the opposite effect. Out of the 22 biochars tested, 11 significantly reduced N2O emissions by an average of 48 percent, seven showed no significant effect and four significantly increased emissions. The strongest increases were observed for biochars made from rice straw, olive mill waste, giant reed and grape stalks under specific production conditions.
“Our results show that biochar should not be treated as a single, uniform material,” said lead author María L. Cayuela. “Its effects on soil nitrogen cycling depend strongly on how it is produced and which chemical properties it carries into the soil.”
To better understand the mechanism, the researchers selected eight biochars for a second experiment and used a 15N gas-flux method to distinguish between N2O and N2 produced during denitrification. This approach allowed the team to see not only how much N2O was emitted, but also whether nitrogen was being fully converted to harmless N2.
The results revealed a striking temperature effect. Biochars produced at 400 °C tended to increase total denitrification by 28 percent while favoring the complete reduction of N2O to N2. In contrast, biochars produced at 600 °C reduced total denitrification by 53 percent, but shifted the remaining denitrification products toward a higher proportion of N2O.
A key chemical clue was found on the biochar surface. The study identified carboxylic groups as the strongest predictor of increased N2O emissions, especially in biochars produced at 600 °C. Biochars with higher electrical conductivity and pH also promoted greater nitrite accumulation, suggesting that certain biochars can disrupt nitrogen transformations in soil.
“This does not mean biochar is a poor mitigation strategy,” said Cayuela. “It means that selecting the right biochar for the right soil is essential. Better characterization can help avoid unintended emissions and improve the climate performance of biochar amendments.”
The findings highlight the need for more precise biochar recommendations in agriculture. Rather than assuming all biochars reduce greenhouse gas emissions, the study suggests that future guidelines should consider production temperature, feedstock and surface chemistry.
By linking specific biochar properties to denitrification products, the research provides a clearer path for designing biochars that support both soil management and climate mitigation.
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Journal Reference: Cayuela, M.L., Spott, O., Pascual, M.B. et al. Key biochar properties linked to denitrification products in a calcareous soil. Biochar 6, 90 (2024).
https://doi.org/10.1007/s42773-024-00386-3
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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.