Variation in Soil Methane Release or Uptake Responses to Biochar Amendment: A Separate Meta-analysis

Agricultural soils play an important role in the atmospheric methane (CH4) budget, where paddy soils can contribute significant CH4 to atmosphere whereas upland soils may act as a source or sink of atmospheric CH4, dependent on soil water conditions. Biochar amendments have effects on soil CH4 production or oxidation processes in individual experiments, but the causative mechanisms are yet to be fully elucidated. To synthesize the response of soil CH4 release or uptake to biochar amendment, we performed a meta-analysis using data from 61 peer-reviewed papers with 222 updated paired measurements. When averaged across all studies, biochar amendment significantly decreased CH4 release rates by 12% for paddy soils and 72% for upland soils, and CH4 uptake rates by 84% for upland soils. Neither soil CH4 release nor uptake responses to biochar amendment were significant in field soils. Nitrogen (N) fertilizer application would weaken the response of soil CH4 release or uptake to biochar amendment. Biochar-incurred decreases in soil CH4 release and uptake rates were the largest in medium-textured soils or neutral-pH soils. Soil CH4 release or uptake responses to biochar were also significantly altered by biochar characteristics, such as feedstock source, C/N ratio, pH, and pyrolysis temperature. The results of this synthesis suggest that the role of biochar in soil CH4 mitigation potential might have been exaggerated, particularly in fields when biochar is applied in combination with N fertilizer.