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Biochar has been recommended as a soil amendment to improve soil fertility and mitigate methane (CH4) emissions from rice cultivation. Its effects, however, vary depending on soil type, biochar characteristics, and application rate. This study was aimed to evaluate the potential of mangrove biochar on CH4 mitigation, soil properties, and the productivity of rice cultivated in a clay loam soil in Thailand. Biochar was used at a rate equivalent to 10 t ha−1 season−1, both with (biochar + fertilizer: BF) and without (biochar alone: BI) fertilizer, for two cultivation seasons. BI reduced CH4 flux at most stages of rice growth. Relative to control soil (no biochar, no fertilizer: CT), BI significantly decreased cumulative CH4 emissions by 21.1% in the first season and 24.9% in the second season. CH4 emissions from BF soil were also less than those from the use of fertilizer alone (FE). Rice grain yield in BI was 7.85% and 14.4% greater than in CT, and in BF by 1.47% and 3.72% greater than FE, in the first and second seasons, respectively. The decrease in CH4 emissions and increase in rice grain yield decreased CH4 emission intensity under biochar treatments. Soil pH, organic carbon, cation exchange capacity, and available nutrients in the soil increased with biochar addition. The soil organic carbon stock was significantly increased by 32.6% in BI and 27.5% in BF after the first season, and by 43.5% in BI and 39.6% in BF after the second season. [ABSTRACT FROM AUTHOR]