Role of pyrochar amendment toward carbon neutrality of rice paddy field by mitigating methane emission: A global evaluation by meta-analysis

Context: Mitigating methane (CH4) emissions from the rice paddy is crucial for preventing global warming and achieving carbon neutrality in agroecosystems. While biochars are effective soil amendments for reducing CH4 emissions, their efficacy varies with different feedstocks, soil properties, climate conditions, and field management. A global evaluation needs to be conducted. Objective: 1358 observations based on 189 studies from 22 countries were collected to establish a meta database, aiming to evaluate the impacts of pyrochars and hydrochars on CH4 emission from paddy fields and compare their effectiveness with that of other amendments, including straw, organic fertilizer, and industrial residue. Methods: The effects of different amendments on CH4 emissions from the rice paddy were assessed using a metaanalysis. Sub-group meta-analysis, multivariate analyses and random forest analyses were used to explore responses to climate factors, amendment properties, and soil properties. The relationship between rice crop yield and CH4 emission under pyrochar amendment was analyzed using linear multiple fitting models. Results and conclusions: Results show that pyrochar amendment significantly reduced CH4 emission by 19.9 % (mean lnRR (effect size) = -0.223, 95 % CI (confidence interval): -0.310 - -0.135) and had more stable mitigation effects than industrial residue amendment. In contrast, straw amendment significantly increased CH4 emission, whereas hydrochar, slow-release fertilizer, organic amendment, and straw amendment had negligible impacts. CH4 emission increased by 45.2 % (mean lnRR = 0.427, 95 % CI: 0.215-0.639) in temperate monsoon regions, with no significant effects in tropical monsoon and subtropical monsoon regions. Pyrochar produced from sawdust was most effective in reducing CH4 emissions by 44.7 % (mean lnRR = 0.593 % CI: -0.837 - 0.348). Linear fitting results indicate that pyrochars produced at 550 degrees C, with 55 % carbon and pH 9, achieved the highest mitigation efficacy. The positive correlation between pyrochar amendment rates and CH4 mitigation, along with the optimal nitrogen fertilizer application rates at 220 kg ha-1, underscores the importance of integrated soil management for sustainability. Additionally, pyrochar amendments pose dual benefits on CH4 mitigation and yield increase, essential for sustainable agriculture by enhancing food production without exacerbating climate change. Significance: These findings provide valuable strategies for applying pyrochars in mitigating greenhouse gas emissions and achieving carbon neutrality in rice-paddy systems.