The molecular composition of soil organic matter is regulated by bacterial community under biochar application

• Py-GC/MS revealed biochar increased SOM molecular diversity and network complexity in soil. • In biochar-amended soil, lignin-derived products increased, while aromatic compounds and lipids decreased. • Biochar enhances the soil life-history strategies of copiotroph bacteria. • The molecular composition of SOM is regulated by bacterial communities. Soil organic matter (SOM) consists of diverse carbon compounds, which are influenced by microorganisms that affect its turnover and stability. However, changes in SOM molecular composition following biochar application and their interactions with the soil bacterial communities remain poorly understood. We aimed to evaluate SOM molecular composition, soil bacterial communities, and carbon cycle functional genes of bacteria in soils treated with biochar using pyrolysis–gas chromatography-mass spectrometry (py-GC/MS) and amplicon sequencing. The py-GC/MS results indicated that biochar increased the molecular diversity and significantly altered the molecular composition of SOM. In biochar-treated soils, the abundance of lignin-derived products increases, while lipids levels decrease. Biochar application shifted the soil bacterial life-history strategy towards copiotrophy, characterised by a higher copiotroph/oligotroph ratio and ribosomal RNA operon copy number. Procrustes analysis revealed that SOM molecular composition was strongly correlated with both the bacterial community and carbon cycle functional genes. Specifically, the SOM composition was closely associated with Gammaproteobacteria, Acidobacteria, and Chloroflexi. Additionally, SOM network analysis indicated that biochar enhanced SOM molecular complexity (i.e., node count, edge count, and average degree) primarily due to the accumulation of lignin-derived products. These findings highlight the potential of biochar to reshape the molecular composition of SOM via microbially mediated processes.