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

Abstract

Purpose It is unclear whether a higher biochar (BC) application rate enhances rice ( Oryza sativa L.) yield and reduces CH 4 emissions. This study investigated changes in rice yield and CH 4 emissions with varying BC application rates. Methods Data on rice yield and CH 4 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 CH 4 emission (by 22.2%) decreased. However, the correlation of BC application rates with rice yield and CH 4 mitigation was not significant, implying that a higher BC application rate did not enhance rice yield and CH 4 reduction. Interestingly, for a data set showing increased rice yield and decreased CH 4 emission by BC, the magnitude of change in the rice yield and CH 4 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 CH 4 mitigation are not additive, probably because of the decreases in the inherent capacity of unit weight of BC to enhance rice yield and reduce CH 4 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 CH 4 mitigation using BC.

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