Food waste is often seen as a problem, but it also represents a valuable resource in the transition toward a circular bioeconomy. A new study shows that a simple adjustment in how biochar is produced can dramatically improve the efficiency of composting food waste digestate, cutting nitrogen loss nearly in half while reducing harmful emissions.
Researchers have discovered that the temperature used to produce biochar plays a decisive role in how effectively it conserves nitrogen during composting. By carefully selecting this temperature, they were able to optimize both environmental performance and compost quality.
"Biochar is not just a passive material that absorbs gases," said the study's corresponding author. "It actively shapes the microbial community in compost, and that determines how nitrogen is retained or lost."
Food waste digestate, a byproduct of anaerobic digestion, is rich in nutrients but difficult to manage. During composting, large amounts of nitrogen are lost as ammonia and nitrous oxide. Ammonia contributes to air pollution, while nitrous oxide is a potent greenhouse gas. These losses reduce the fertilizer value of compost and create environmental concerns.
To address this challenge, the research team produced hardwood biochar at three different temperatures, 300 degrees Celsius, 400 degrees Celsius, and 800 degrees Celsius. They then added the biochar to composting systems and tracked nitrogen transformations, gas emissions, and microbial activity.
The results revealed a clear trade-off. Biochar produced at lower temperature was highly effective at reducing ammonia emissions, cutting them by more than 39 percent. This was linked to the presence of oxygen-containing functional groups that enhance ammonium adsorption and stimulate microbial nitrification.
In contrast, high-temperature biochar performed best at reducing nitrous oxide emissions, achieving a reduction of nearly 48 percent. Its highly porous structure improved oxygen diffusion in compost, suppressing microbial processes that generate nitrous oxide.
However, neither extreme provided the best overall outcome. The breakthrough came with biochar produced at 400 degrees Celsius, which achieved the optimal balance between these competing processes. This treatment reduced total nitrogen loss by 46.3 percent compared to the control, the highest performance among all tested conditions.
According to the study, this balance arises because medium-temperature biochar simultaneously supports beneficial microbial communities and provides sufficient adsorption capacity. It enhances nitrification while avoiding excessive stimulation of incomplete denitrification, which can lead to nitrous oxide production.
The findings also highlight the importance of microbial ecology in composting systems. Biochar was shown to act as a habitat that selectively enriches key nitrogen-cycling microorganisms, including ammonia-oxidizing bacteria and nitrite-oxidizing bacteria. These microbes play a central role in converting nitrogen into stable forms that remain in the compost.
Beyond improving compost quality, the approach offers broader environmental benefits. Reducing nitrogen loss helps retain nutrients in agricultural systems, while lowering greenhouse gas emissions supports climate mitigation efforts.
The researchers note that biochar produced at moderate temperatures is also more practical from an energy and economic perspective, making it suitable for large-scale applications.
"This work provides a clear framework for designing biochar-based strategies to improve composting," the author said. "By tuning pyrolysis temperature, we can steer microbial processes and achieve both environmental and agronomic benefits."
The study provides new insights into how engineered carbon materials can be used to enhance sustainable waste management. Future research will focus on scaling up the approach and further refining biochar properties to target specific environmental outcomes.
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Journal Reference: Li, D., Zhou, J., Liang, J. et al. Nitrogen conservation by hardwood biochar during food waste digestate composting: pyrolytic temperature dictates microbial mechanisms. Biochar 8, 75 (2026).
https://doi.org/10.1007/s42773-026-00588-x
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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.
