How does biochar reduce N2O emission from soil? - a review of mechanisms

Agricultural soils are a major source of nitrous oxide (N 2 O), a potent greenhouse gas. Biochar application has emerged as a promising mitigation strategy, but our understanding of the mechanisms behind its effects and the interplay between biochar properties, soil conditions, and microbial processes is still incomplete. This review synthesizes recent advances of our understanding of how biochar influences nitrogen transformations and N 2 O production, consumption and transport in soils and sediments. Biochar properties, governed by feedstock type and pyrolysis conditions, determine its surface area, porosity, functional groups, pH, and aromaticity, which in turn influence nitrogen cycling pathways involving both production and consumption of N 2 O. Mechanistically, biochar reduces N 2 O emissions primarily by altering the environmental conditions that regulate N 2 O production and consumption pathways rather than by directly suppressing a single microbial process. The most consistently supported mechanisms include increases in soil pH promoting N 2 O reduction to N 2 , changes in oxygen diffusion and redox conditions affecting the balance between nitrification and denitrification, modulation of carbon and nitrogen substrate availability, and electron transfer-mediated enhancement of denitrification. Despite substantial experimental evidence, knowledge gaps remain regarding the relative importance of mechanisms under specific soil contexts, the roles of critical alternative pathways (e.g., co-denitrification, chemodenitrification), and the electrochemical properties of biochar. Future research should integrate isotopic tracers, imaging, microsensors, light-based techniques and molecular tools, systematically characterize the electron donating and accepting capacity (EDC/EAC), and conduct cross-comparisons across diverse biochars to identify traits that consistently reduce N 2 O emissions. A mechanistic framework linking biochar properties to soil processes is essential to guide the design and targeted application of biochar for effective greenhouse gas mitigation in agriculture.