30 Years of Carbon Stability: Soil Organic Carbon Dynamics in the Pfälzerwald and Bienwald

Soil organic carbon (SOC) represents roughly 50% of terrestrial carbon stocks and can persist in soils for millennia, making soils an important long term carbon sink. However, with ongoing climate change and increasing disturbances, carbon rich soils may shift from carbon storage to net carbon release. In Rheinland-Pfalz, Germany, the largest SOC stocks occur in forest soils. Yet it remains unclear which forest ecosystems, management regimes, and site conditions promote long term stability of SOC. We investigated SOC stability using National Forest Soil Inventory samples from 1990, 2005, and 2020 in the acidic, sandy soils of the Pfälzerwald and Bienwald. SOC stability was assessed by density fractionation of organic matter into the free light fraction (fLF), occluded light fraction (oLF), and heavy fraction (HF), representing increasingly stable carbon pools. In the topsoil, concentrations of both fLF (10.06 → 13.17 mg C g⁻¹) and HF (5.17 → 8.54 mg C g⁻¹) increased, indicating enhanced carbon inputs and stabilization, while oLF concentrations remained constant or slightly declined. In the subsoil, HF amounts remained largely constant across all inventories, and fLF and oLF showed only minor fluctuations. Fungal communities exerted a strong and persistent influence on carbon dynamics in both the labile and stable SOC fractions, highlighting their key role in long term SOC formation and stabilization. Precipitation primarily affected SOC dynamics in the oLF fraction.These results demonstrate that SOC stabilization in forest soils is depth dependent and driven by biological controls interacting with site conditions and disturbance history.