Drivers for soil organic carbon stabilization in Elbe River floodplains

Background: Floodplains play an important role in the global carbon (C) cycle, particularly due to their large soil organic carbon (SOC) storage potential. However, a heterogeneous microrelief and anthropogenic landscape modifications complicate the understanding of stabilization processes of SOC in floodplains. Aim: Determining the predominant drivers for SOC stabilization in Elbe River floodplain soils. Methods: We measured SOC density fractions, microbial biomass, and mineralization characteristics in top- and subsoils of eight floodplain sites of the lower middle Elbe River. Results: The heavy fraction (HF) was the most important SOC pool, with a contribution of >64% at both depth intervals. With soil depth, HF pool size increased and the occluded light fraction (oLF) decreased, whereas the free LF (fLF) stayed the same. The contribution of the HF to SOC was positively related to fine texture (R ² = 0.64). Mineralizable C was negatively related to fine texture at both depth intervals. Both results suggest a positive effect of fine texture on SOC stabilization. The metabolic quotient was related to the amount of available SOC in the topsoil, but no relation was found in the subsoil. However, in top- and subsoil, the mineralization rate constant was positively related to the C/N ratios of the fLF and the oLF, indicating that the quality of fresh plant litter is an important energy source for microbial mineralization. Conclusion: Sedimentation of fine-textured material is the most important driver for SOC stabilization rather than fresh plant litter input. Thus, SOC stabilization strongly depends on relief, flooding, and sedimentation.