Effects of different biochar application rates on CH4 emissions and Microbial Carbon Pump (MCP) mediated carbon sequestration in paddy fields: a quantitative modeling approach.

Biochar amendment is a proven strategy for mitigating methane (CH4) emissions and enhancing soil carbon sequestration in rice paddies. However, the depth-dependent dynamics of microbial carbon pump (MCP)-driven recalcitrant organic carbon (ROC) formation and the underlying microbial mechanisms remain poorly characterized, particularly in the purple paddy soils of the central Sichuan Basin, China. To address this gap, we developed a novel quantitative MCP-driven ROC model and conducted a field experiment with four biochar treatments (CK, C2, C4, and C6 t ha-1) to investigate CH4 emissions, methane-cycling microbial community, and soil carbon fractions across a 0-80 cm profile. Biochar amendment significantly reduced CH4 emissions by 57.88-84.51% (peaking at 6 t ha-1) and increased methanogen and methanotroph diversity by 1.30-1.66 times. Although the concentrations of soil organic carbon (SOC), dissolved organic carbon (DOC), microbial biomass carbon (MBC), and absolute ROC decreased significantly with depth, the ROC/SOC ratio consistently increased. Crucially, our quantitative model revealed that biochar stimulated both the in vivo turnover and ex vivo modification pathways of the MCP by supplying essential labile substrates, explaining up to 92% of the variance in deep-soil carbon fractions. In conclusion, this study demonstrates that biochar serves as an effective dual-action strategy: mitigating CH4 emissions via microbiome regulation while enhancing long-term carbon sequestration through intensified MCP-driven ROC formation across the entire soil profile.