Luxury application of biochar does not enhance rice yield and methane mitigation: a review and data analysis

Purpose: It is unclear whether a higher biochar (BC) application rate enhances rice (Oryza sativa L.) yield and reduces CH₄ emissions. This study investigated changes in rice yield and CH₄ emissions with varying BC application rates. Methods: Data on rice yield and CH₄ emission from paddies amended with or without BC were collected from the literature, and the biochar effects were analyzed using the data set. Results: Across the biochar application rate from 2 to 48 t ha^-1, the rice yield increased (by 10.8%) while the area-scaled (by 14.4%) and yield-scaled CH₄ emission (by 22.2%) decreased. However, the correlation of BC application rates with rice yield and CH₄ mitigation was not significant, implying that a higher BC application rate did not enhance rice yield and CH₄ reduction. Interestingly, for a data set showing increased rice yield and decreased CH₄ emission by BC, the magnitude of change in the rice yield and CH₄ mitigation per unit weight of BC (1 t ha^-1) decreased with an increase in the BC application rate. These results suggest that BC effects on rice yield and CH₄ mitigation are not additive, probably because of the decreases in the inherent capacity of unit weight of BC to enhance rice yield and reduce CH₄ emission, which might be caused by the adverse effects of toxic compounds contained in BC, losses of BC, and a higher degree of nutrient immobilization by BC. Conclusions: Annual BC application at a low rate (e.g., 2 t ha^-1) rather than a luxury application may be an effective and economical strategy for long-term rice yield enhancement and CH₄ mitigation using BC.