Biochar-driven soil carbon sequestration: priming effects and emission reduction

Biochar is widely used in agriculture and pollution remediation, but its effect on soil carbon cycling balance remains uncertain. Due to the growing interest in biochar-induced priming effects, which can inhibit or promote mineralization that affects carbon cycling, assessing its carbon stabilization and mechanisms is essential. This carbon-rich byproduct of biomass pyrolysis enhances carbon sequestration and reduces CO₂ emissions through its stable aromatic structure and soil interactions, though its efficiency is limited by environmental, physicochemical and technological thresholds. Discrepancies among short-term experimental/model predictions and natural conditions suggest that current research may underestimate carbon oxidation risks by over-reliance on laboratory conditions and neglecting the complexity of natural environments. Negative priming effects arise only under specific conditions. Furthermore, the trade-off between biochar pyrolysis technology and energy recovery poses a significant challenge. This review critically analyzes the key mechanisms through which biochar stabilizes soil organic carbon, in accordance with the bidirectionality of priming effects. Key mechanisms include: (i) physical protection of organic-mineral complexes and soil aggregation; (ii) chemical stability of aromatic polymer condensation and oxidized biochar; and (iii) microbial metabolism and transformations of plant-derived carbon. Future research should clarify the molecular transformation mechanisms of soil carbon from various sources after biochar application, using diverse analytical techniques. Additionally, it should focus on developing biochar application systems tailored to different field conditions and regional soil characteristics. This review provides a theoretical framework for optimizing biochar's carbon sequestration pathways and establishes a scientific foundation for the development of technologies for carbon reduction.