Coupled mineral and microbial carbon pumps govern soil organic carbon accumulation in croplands at a large spatial scale

Soil organic carbon (SOC) exhibits substantial spatial variability across broad geographic regions due to interactions among climate, soil properties, and land management. In croplands, SOC accumulation is further shaped by cropping systems and residue management, yet the general patterns and mechanistic drivers of SOC formation across multiple carbon (C) pools and sources remain poorly understood. Here, we analyzed cropland soils along a Mollisol transect in Northeast China to elucidate the key mechanisms and pathways underlying SOC accumulation. The results showed that both microbial necromass and mineral-associated organic C (MAOC) declined with increasing mean annual temperature and collectively served as the predominant contributors to SOC in croplands under different cropping systems. Microbial necromass accumulation was primarily regulated by microbial properties (61% of explained variance) and soil organic matter (SOM) chemical recalcitrance (31%), whereas MAOC accumulation was mainly driven by microbial necromass (50% of explained variance) and mineral protection capacity (41%). Across the Mollisol transect, microbial necromass positively responded to microbial biomass C and fungal community dissimilarity, but decreased with enhanced SOM chemical recalcitrance, as indicated by elevated aromaticity, alkyl C/O-alkyl C ratio, and hydrophobicity index. Furthermore, MAOC accumulation followed a two-step pathway involving microbial necromass inputs through in vivo microbial turnover and subsequent mineral stabilization by association with poorly crystalline Fe/Al oxides and exchangeable Mg 2+ . Together, microbial and mineral C pumps jointly regulated SOC accumulation, with the mineral C pump exerting a dominant role. These findings highlight the importance of explicitly integrating coupled microbial and mineral C pumps mechanisms into SOC stabilization frameworks. We demonstrate that agricultural management practices that enhance microbial activity, necromass production, and mineral protection capacity can substantially increase SOC sequestration in cropland ecosystems.