Changes in organic carbon fractions and sources in deltaic topsoil and subsoil layers: autochthonous and allochthonous inputs

River deltas are often considered as hotspots of atmospheric carbon regulation, but few studies have paid attention to the differences between topsoil and subsoil for sequestration. Here physical chemical stabilization mechanisms sources soil organic (OC) accumulation in different layers a major river delta (Yellow delta) were studied by fractionation isotopic analysis. We found that topsoil, conversion from tidal flat wetland substantially increases OC 84%, whereas cotton cultivation decreases 66%. Wetland with high content is characterized substantial plant-derived mineral fractions. In subsoil, variation controlled yellow silt layer (YSL)–red clay (RCL) sequence. Although weathering intensity RCL much higher than YSL, age younger RCL, indicating highly reactive, secondary minerals iron oxides capable stabilizing transportation, deposition post-pedogenic alteration. 14C activity demonstrates modern autochthonous main component topsoil. contrast, millennia-old allochthonous layers, contributing 4–24% total bulk OC. The δ13C values fractions (−24.3 −21.4‰ intra-aggregate particulate matter fraction, intra-microaggregate free faction) samples (~ −22‰) likely imply contribution aged Yellow delta. This study offers direct confirmation influenced land-use types pedological properties characteristic layers. These findings suggest biogeochemical alteration matrix controls sediment–tidal flat–wetland–farmland continuum. Highlights Land-use features control storage respectively. Reclamation strongly all proportion fSC rOC. Soil distinct texture compositions lead stratification subsoil. red both fresh