Biochar–Straw Interaction Drives Soil Carbon Accrual by Modulating Microbial Metabolic Pathways

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Straw and biochar additions can enhance long-term carbon storage, yet how their interactions regulate carbon mineralization and allocation remains unclear. We conducted a 360-day microcosm incubation employing a reciprocal 13C-labeling design that paired 13C-labeled straw with unlabeled biochar and unlabeled straw with 13C-labeled biochar and used single-cell Raman spectroscopy and 13C-labeled cell metagenomics. At day 360, biochar reduced cumulative straw-derived 13CO2 from 1391 to 1199 mg C kg–1 soil, while increasing straw-C allocation to fulvic acid (FA) and humic acid (HA) fractions. This shift was associated with enrichment of bacterial taxa such as Chitinophagales and increased abundance of genes involved in phospholipid metabolism, the phenylacetyl-CoA pathway, and potential polyphenol-related processing. Conversely, straw increased biochar-derived dissolved organic carbon from 1.5 to 5.5 mg C kg–1 soil and cumulative 13CO2 release from 17 to 23 mg C kg–1 soil, increasing FA and HA allocation. This shift was associated with the enrichment of genes involved in oxidative degradation, membrane transport (for substrate uptake), exopolysaccharide (EPS) formation, and quinone biosynthesis. Overall, the biochar–straw interaction redirected exogenous carbon from rapid mineralization toward humic substance associated soil organic matter fractions, thereby enhancing soil carbon accrual.