Biochar modulates microbial carbon metabolism to mitigate global warming potential during composting

Carbon loss via CO₂ and CH₄ emissions reduces the organic carbon available for humification, which may negatively affect compost quality while exacerbating environmental pollution. Herein, this study investigated the impacts of biochar (e.g., rice husk biochar (RHB) and sawdust biochar (SDB) as composting additives on CO₂ and CH₄ emissions, as well as the underlying microbial mechanisms. The results demonstrated that, comparing to the control (CK), RHB and SDB reduced cumulative CO₂ emissions by 11.38% and 16.30%, respectively (both P < 0.05), whereas they increased CH₄ emissions by 45.35% (P < 0.05) and 81.04% (P < 0.01), respectively. Overall, the 100-year global warming potentials (GWP-100s) of the RHB and SDB treated groups were decreased by 15.91 g CO₂e·kg^-1 and 19.29 g CO₂e·kg^-1, respectively. Redundancy analysis (RDA) and partial least squares path modeling (PLS-PM) revealed the total organic carbon (TOC) to be the key environmental regulatory factor. Specifically, TOC was significantly negatively correlated with CO₂-producing genes and aerobic methane oxidation genes, yet it exhibited a positive correlation with methanogenic genes which underlines the up-regulation of CH₄ emissions from the biochar-treated composting systems. The addition of biochar increased the TOC of the compost systems, which induced microbial community shift from r-strategy to K-strategy, inhibited CO₂ production, yet modulated methanogenesis. This cascade of effects reduces the net GWP of compost via microbial-mediated mechanisms. The findings of this study provided a critical theoretical foundation and a practical guideline for optimizing the biochar-manure co-composting process to reduce overall greenhouse gas emission from composting.