Research progress and prospects for using biochar to mitigate greenhouse gas emissions during composting: A review

Biochar possesses a unique porous structure and abundant surface functional groups, which can potentially help mitigate greenhouse gas (GHG) emissions from compost. This review summarizes the properties and functions of biochar, and the effects of biochar on common GHGs (methane (CH₄), carbon dioxide (CO₂), and nitrous oxide (N₂O)) and ammonia (NH₃, an indirect GHG) during composting. Studies have shown that it is possible to improve the mitigation of GHG emissions during composting by adjusting the biochar amount, type of raw material, pyrolysis temperature, and particle size. Biochar produced from crop residues and woody biomass has a greater effect on mitigating CH₄, N₂O, and NH₃ emissions during composting, and GHG emissions can be reduced significantly by adding about 10% (w/w) biochar. Biochar produced by high temperature pyrolysis (500-900 °C) has a greater effect on mitigating CH₄ and N₂O emissions, whereas biochar generated by low temperature pyrolysis (200-500 °C) is more effective at reducing NH₃ emissions. Interestingly, adding granular biochar is more beneficial for mitigating CH₄ emissions, whereas adding powdered biochar is better at reducing NH₃ emissions. According to the current research status, developing new methods for producing and using biochar (e.g., modified or combined with other additives) should be the focus of future research into mitigating GHG emissions during composting. The findings summarized in this review may provide a reference to allow the establishment of standards for using biochar to mitigate GHG emissions from compost.