Is initial soil organic carbon more important than texture for the fate of carbon inputs into temperate agricultural soils?

The accrual of stabilized soil organic carbon (SOC) can mitigate the atmospheric CO₂ concentration and thus climate change. Organic carbon in the fine silt and clay size fraction (OC_fine) is typically mineral-associated and thus considered relatively stable compared to the coarse fraction. The SOC saturation concept suggests that this pool has a limited storage capacity, for which the fine soil particle content constrains SOC sequestration. Therefore, fine-textured soils with low OC loading of the fine fraction are thought to have a greater potential to stabilize additional OC than soil with high OC loading and coarse-texture due to their higher available storage space. Here, we assessed soils’ potential to stabilize additional OC_fine using 21 temperate agricultural soils. The soils were selected from the archive of the German Agricultural Soil Inventory to analyze SOC gradients (0.7-10.2 %) in three texture classes (sandy, loamy, clayey). After a two-years incubation, we investigated the recovery of ¹³C labeled litter in two size-based fractions: OC_coarse (>20µm) and OC_fine (<20 µm). Our results show that the litter-derived OC retention increased significantly with initial SOC content and fine fraction OC loading. This was primarily driven by the OC_coarse fraction, which indicated that less added litter was decomposed/transformed in the presence of sufficient SOC. 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 analysis revealed that microscale organic matter patches drove litter-derived OC formation. We found large parts of litter-derived SOC allocated with likely pre-exisiting SOC patches suggesting a high importance of organo-organic interactions. All soils also had new OC_fine on mineral-dominated surfaces. Furthermore, five out of six soils were still dominated by bare mineral surfaces, despite partly very high SOC contents. Taken together, those findings revealed that OC loading of the fine fraction or soil texture are not the major limiting factors of new OC_fine formation. Instead, initial SOC content have a positive effect on litter-derived OC retention by retarding its mineralization. This feedback of pre-existing SOC on the dynamic and fate of new OC_fine should be studied more closely from a microbiological perspective and considered in SOC models.