Biochar mitigates methane emissions from organic mulching in urban soils: Evidence from a long-term mesocosm experiment

Methane (CH₄), a potent greenhouse gas (GHG) with high global warming potential, significantly contributes to urban GHG emissions. Organic mulching, commonly practiced in urban forestry, may promote CH₄ emissions via anaerobic decomposition; yet its impact on the urban carbon budget has largely been unexamined. Biochar has shown promise in mitigating CH₄ emissions in agricultural soils, but its effectiveness in urban mulched systems remains unknown. This study employed a mesocosm experiment to investigate the effects of organic mulches (woodchips and bark) and biochar amendments (50 t/ha), applied either on the surface (top-dressed) or incorporated (mixed), on fluxes of CH₄, CO₂, and H₂O. Fluxes were measured using an off-axis integrated cavity output spectroscopy analyzer. Results indicate that mulched soils emitted CH₄ at 1.0-1.5 nmol m⁻^2.s⁻¹, whereas biochar amendments promoted CH₄ uptake, in the case of both woodchips (-1.65 ± 1.03 nmol m⁻^2.s⁻¹) and bark mulch (-0.49 ± 0.16 nmol m⁻^2.s⁻¹) by the second year. Mixed treatments showed greater CH₄ uptake; for instance, incorporating biochar into bark mulch led to a mean CH₄ uptake (-2.02 ± 1.02 nmol m⁻^2.s⁻¹), nearly fivefold greater than controls. While mulch additions reduced water loss and increased soil organic carbon-factors contributing to CH₄ emissions-biochar amendments increased CO₂ emissions by 26.7%-121.1%. Biochar-mediated CH₄ uptake correlated with substrate pH, bulk density, and C:N ratio, suggesting enhanced microbial activity and increased CO₂ release. Overall, findings indicate that biochar, combined with organic mulching, can serve as an effective GHG mitigation strategy, informing climate-smart soil management in urban landscapes.