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

Abstract 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 carbon sequestration. Here physical and chemical stabilization mechanisms and sources of soil organic carbon (OC) accumulation in different soil layers of a major river delta (Yellow River delta) were studied by fractionation and isotopic analysis. We found that in topsoil, conversion from tidal flat to wetland substantially increases OC by 84%, whereas cotton cultivation decreases OC by 66%. Wetland with a high soil OC content is characterized by substantial accumulation of plant‐derived OC and OC in mineral fractions. In subsoil, the OC variation is controlled by the yellow silt layer (YSL)–red clay layer (RCL) sequence. Although the weathering intensity of the RCL is much higher than that of the YSL, the OC age is younger in the RCL, indicating that the highly reactive, secondary minerals and iron oxides are capable of stabilizing younger organic carbon in transportation, deposition and post‐pedogenic alteration. 14 C activity demonstrates that modern autochthonous OC is the main component of OC in wetland topsoil. In contrast, millennia‐old allochthonous OC is the main component of OC in the subsoil layers, contributing 4–24% of total bulk soil OC. The higher δ 13 C values of carbon fractions (−24.3 to −21.4‰ for the intra‐aggregate particulate organic matter fraction, intra‐microaggregate silt and clay fraction, and free silt and clay faction) and bulk samples (~ −22‰) are likely to imply the contribution of aged OC in the Yellow River delta. This study offers direct confirmation that soil OC fractions and sources are influenced by different land‐use types and pedological properties in characteristic soil layers. These findings suggest that biogeochemical alteration of the soil matrix controls OC stabilization mechanisms in the river sediment–tidal flat–wetland–farmland continuum. Highlights Land‐use types and pedological features control OC storage in topsoil and subsoil, respectively. Reclamation strongly decreases content of all OC fractions and the proportion of fSC and rOC. Soil layers with distinct texture and mineral compositions lead to OC stratification in subsoil. The red clay layer in subsoil is capable of stabilizing both fresh and aged OC.