Organic amendment increases soil carbon sequestration by altering carbon stabilization pathways within soil aggregates

Organic amendment has been proven to be an effective strategy for increasing soil carbon (C) sequestration. However, the pathways which different organic amendments regulate C accumulation and stabilization processes may vary due to their differing inherent properties. Field experiments with four treatments, viz. CK, no fertilizer; CF, conventional chemical fertilizer; CFM, chemical fertilizer with manure; CFB, chemical fertilizer with biochar were conducted to elucidate the differential mechanisms governing soil aggregate C flow pathways mediated by organic amendments. The results demonstrated that CFM and CFB treatments increased SOC content by 12 % and 21 % respectively compared to CF treatment. Specifically, biochar increased C stabilization through structural optimization of aggregates (>2 mm macroaggregates increased by 4 %) and improved C sequestration across all aggregate sizes (5–24 % increase in SOC content). Nuclear magnetic resonance (NMR) analysis revealed distinct stabilization pathways: CFB preferentially elevated aromatic C proportions (15 %-29 % increase), while CFM promoted alkyl C accumulation (38 %-64 % increase). Biochar directly introduced recalcitrant aromatic compounds, whereas manure facilitated microbial conversion of labile C to alkyl C. δ¹³C fractionation analysis further delineated distinct stabilization pathways: CF accelerated native SOC depletion through preferential utilization of labile components, biochar facilitated C stabilization in via physicochemical protection, while manure balanced metabolic partitioning and kinetic fractionation during microbial processing. These findings underscore biochar's superiority in long-term C sequestration via physicochemical stabilization and manure's efficacy in optimizing C turnover efficiency, providing mechanistic foundations for precision organic amendment strategies in sustainable soil management.