Increased Retention of Litter-Derived Organic Carbon With Increasing Initial Carbon Content in Temperate Agricultural Soils

Stabilized soil organic carbon (SOC) accrual plays a crucial role in long-term atmospheric CO₂ sequestration. The organic carbon in the fine silt and clay size fraction (OC_fine) is typically mineral-associated and thus relatively stable. However, the SOC saturation concept suggests that the OC_fine has limited capacity for additional carbon (C) storage, thereby constraining further C sequestration. Low-OC and fine-textured soils are thought to have greater potential to stabilize additional OC than High-OC and coarse-textured soils due to their higher available storage space. Here, we assessed soils' potential to stabilize additional OC using 21 temperate agricultural soils, varying in SOC (0.7%–10.2%), silt + clay content (32%–92%), and OC loading of fine fraction (17–135 g C kg⁻¹). We investigated the decomposition and recovery of uniform ¹³C labeled litter after 2 years in two size-based fractions: OC_coarse (> 20 μm, the OC associated with coarse silt and sand) and OC_fine (< 20 μm). Litter-derived OC retention increased significantly with initial SOC content and fine fraction OC loading, primarily driven by the OC_coarse fraction, which indicated that less added C was utilized by microbes when enough C was already abundant. In contrast, litter-derived OC_fine formation was negatively correlated with initial SOC and fine fraction OC loading. However, when normalized to the amount of actually decomposed litter, initial SOC and texture did not significantly affect the efficiency of OC_fine formation. NanoSIMS showed litter-derived OC forming at distinct microscale patches, partly overlapping with OM- and mineral-dominated sites. Both findings together revealed that initial SOC content in the studied range, OC loading of the fine fraction, or even soil texture may not be major limiting factors of new OC_fine formation. Instead, increasing initial SOC content appeared to have a positive effect on litter-derived OC retention by retarding its mineralization.