Abstract: Pyrolysis of waste biomass to produce biochar is one of the critical technologies for recycling and reuse of biomass carbon, which benefits carbon neutrality. Pyrolysis of waste plant biomass into biochar reduces CO₂ emissions derived from the natural decomposition of biomasses. In addition, a rational application of biochar can mediate microbial activities and benefit the carbon utilization efficiency of microorganisms, reducing the mineralization of soil organic carbon and the resultant CO₂ emissions. Moreover, biochar application can improve soil aeration, which is conducive to increasing the biological oxidation of CH₄. Since biochar has a porous structure, high specific surface area, and abundant functional groups, it can adsorb and fix carbon resources that are easily lost such as CO₂-carbon and dissolved organic carbon in soils, increasing soil carbon pool and quality. Furthermore, a rational application of biochar also benefits photosynthesis and regulating genes involved in plant metabolism and growth, which contributes to increasing plant biomass and crop yields. Therefore, biochar application has both environmental and economic benefits. To sum up, biochar shows great potential to serve “carbon neutrality” in an agroecosystem with the two brushes of “aboveground growth promotion” and “underground carbon-pool promotion”. However, the endogenous pollutants, heterogeneity, and persistence of biochar suggest that biochar is likely to have long-term ecological and environmental risks. How to produce environment-friendly biochar, how to select the appropriate type and application dosage of biochar, and how to apply biochar adapting to local soil conditions are still important challenges to date. In future research, the mechanisms of biochar increasing soil carbon sequestration and the resultant long-term ecological effects should be further investigated. In addition, since biochar shows excellent effects on plant growth and soil improvement, kinds of “functional soil” based on biochar are suggested to be developed. A standardized and large-scale biochar production should be expanded to achieve efficient integration of waste biomass resources and the green production of biochar. A criterion for rational application of biochar will guarantee the sustainable service of biochar for “carbon neutrality”.