Synergistic application of biochar with organic fertilizer enhances soil carbon sequestration by optimizing...

Mineral-associated organic matter (MAOM) constitutes the dominant long-term carbon pool for soil organic carbon (SOC) sequestration, yet how MAOM formation pathways respond to soil management strategies remains unclear. We conducted a field experiment with four treatments: no addition (Control), biochar (BC) applied alone, or combined with low-dose (BC + LF) and high-dose (BC + HF) organic fertilizer (OF). Enzyme activity and metagenomic analyses revealed that different amendments enhanced MAOM formation by facilitating microbial carbon pump (MCP)-mediated turnover pathway. BC + LF appropriately improved MCP by elevating multiple enzyme activities (57.7%-198.8%), enriching r-strategists (e.g., Pseudomonadota), and shifting microbial communities toward resource acquisition, while simultaneously preserving microbial community structure to maximize MAOM formation (37.3%). In contrast, BC alone marginally promoted recalcitrant carbon turnover, whereas BC + HF overstimulated labile carbon turnover. Both treatments limited MAOM accumulation, resulting in only 18.6% and 22.2% increases, respectively. Given the dual role of dissolved organic matter (DOM) as microbial substrate and MAOM precursor, we analyzed DOM characteristics and DOM-microbial interactions to reveal two pathways for MAOM formation: direct sorption of aromatic compounds and microbial transformation of DOM molecules to highly unsaturated compounds (HUCs). A DOM "abundance-transformation" indicator-based approach was applied to estimate the proportional contributions of these two pathways. The amendments increased microbial contributions from 24.3% to 77.1%, with peak MAOM accumulation occurring when microbial contributions reached a critical threshold of 56.2%. Overall, our findings advance mechanistic understanding of how organic amendments regulate microbial turnover to enhance stable carbon pools and highlight that optimizing the balance of MAOM formation pathways can maximize SOC sequestration.