Objective Plant-derived residue carbon (PRC) is a core component of soil organic carbon (SOC), and its transformation process directly determines the stability of farmland carbon pools. Biochar has been widely applied to enhance soil carbon sequestration capacity, yet its regulatory mechanism for plant-derived residue carbon remains unclear.

Methods This study was conducted in farmland of the Yellow River Delta, with ten biochar treatments established using different application rates (0, 4, 8, and 12 t/hm²) and frequencies (annual, biennial, one-time application) to analyze their effects on SOC, lignin phenols, and lignin phenol degradation.

Results All biochar treatments significantly increased SOC content by 15.2% to 52.2%, compared to the control. However, the SOC increase in most treatments was not attributable to the promotion of PRC transformation. The regulation of lignin phenols by biochar exhibited strict application rate-frequency specificity. Only annual application of 8 t/ha and biennial application of 4 t/hm² significantly increased total lignin phenol content by 30.2% and 36.6%, respectively. In contrast, the biennial application of 12 t/hm² biochar and one-time biochar application treatments significantly increased the acid/aldehyde ratios of vanillyl phenol and syringyl phenol monomers, indicating accelerated lignin phenol degradation and reduced PRC retention.

Conclusions Biochar regulates the transformation and retention of PRC in farmland soils of the Yellow River Delta through the coupled effects of application rate and frequency. Considering environmental and economic benefits, biennial application of 4 t/hm² biochar effectively maintains the balance between degradation and accumulation of lignin phenols and enhances SOC stability. The findings provide a theoretical basis for regional farmland carbon sequestration and optimizing biochar application strategies.