Food waste recycling is a growing priority for cities seeking to cut landfill use and build a circular bioeconomy. Anaerobic digestion can turn food waste into renewable energy, but it also leaves behind a wet, nutrient-rich residue known as food waste digestate. Composting this digestate can return nutrients to soil, yet the process often loses valuable nitrogen as ammonia and nitrous oxide, reducing compost quality and contributing to air pollution and climate change.
A new study published in Biochar shows that the temperature used to produce biochar plays a decisive role in controlling these nitrogen losses. The research team found that hardwood biochar produced at 400 °C reduced total nitrogen loss by 46.3% compared with composting without biochar, outperforming biochars made at 300 °C and 800 °C.
"Food waste digestate is rich in nitrogen, but keeping that nitrogen in the compost is a major challenge," said corresponding author Professor Jonathan W. C. Wong. "Our study shows that biochar is not just a passive sponge. Its production temperature determines how it interacts with microbes and nitrogen pathways during composting."
The team compared three hardwood biochars produced at 300 °C, 400 °C, and 800 °C during 42 days of laboratory-scale food waste digestate composting. The results revealed a clear trade-off. Biochar made at 300 °C was most effective at reducing ammonia emissions, cutting ammonia loss by 39.2% compared with the control. This lower-temperature biochar retained more surface functional groups and had a higher cation exchange capacity, which likely helped capture ammonium and ammonia.
However, the same treatment was linked to stronger nitrification and enrichment of microbes carrying denitrification genes, which may increase the risk of nitrous oxide formation. Nitrous oxide is a potent greenhouse gas, making this trade-off important for climate-smart composting.
At the other end of the temperature range, biochar made at 800 °C achieved the strongest reduction in nitrous oxide emissions, lowering nitrous oxide loss by 47.5% compared with the control. Its higher surface area and more porous structure likely improved oxygen diffusion and suppressed incomplete denitrification, a microbial process that produces nitrous oxide.
The most balanced performance came from the middle-temperature biochar. The 400 °C biochar combined useful adsorption properties with favorable microbial regulation, reducing both major nitrogen-loss pathways enough to deliver the greatest overall nitrogen conservation.
"This work helps move biochar use from a general amendment toward a more targeted technology," said first author Dongyi Li. "By choosing the right pyrolysis temperature, composting facilities can better control nitrogen loss, improve compost value, and reduce environmental impacts."
The study also found that biochar accelerated compost maturity. Biochar-amended treatments reached the seed germination maturity threshold much earlier than the control, and final compost maturity indicators were improved. This suggests that properly selected biochar could not only conserve nutrients but also support faster production of stable, plant-friendly compost.
The findings provide a practical message for food waste management: biochar performance depends on how it is made. For food waste digestate composting, hardwood biochar produced at around 400 °C may offer the best balance between nutrient retention, greenhouse gas mitigation, and compost quality.
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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.
