Sensitivity of soil organic carbon stabilization indicators to 24 years of land-use change across soil depth

Soil organic carbon (SOC) and its dynamics are sensitive to changes in land management while assessments of SOC dynamics rely on different indicators of SOC stabilization and are often restricted to topsoil. Here, we evaluated six indicators of SOC stabilization along a 70 cm soil profile under long-term land-use change. Using an agronomic experiment including an unmanaged meadow and two cropland treatments, we quantified isotopic signatures (14C and δ13C), size (mineral-associated OC; MaOC) and thermal (Rock-Eval® and PARTYSOC-derived centennially stable C; CS) fractions, and biochemical composition (amino sugar-derived microbial necromass C; MNC, and glomalin-related soil proteins; GRSP). Isotopic signatures and thermal analysis indicated older SOC (+∼5000 years), a decreasing influence of fresh C (+0.7 ‰ δ13C) and higher proportion of CS (by 75 %) with depth. In the cropland compared to the meadow, mean SOC age increased by ∼ 250 years, δ13C was enriched by 0.75 ‰ and CS was 27 % higher. The proportion of MaOC reflected a slight increase in SOC stabilization with depth (6 %) but decreased in the cropland compared to the meadow (−5%). The proportions of the two biochemical indicators to total OC decreased with depth (−67 % for MNC and − 78 % for GRSP), following the same trend as bulk SOC, while the proportion of MNC decreased (−15 %) and GRSP increased but with very high uncertainties (37 ± 20 %) in the cropland compared to the meadow. Our results suggest that different indicators likely represent SOC stabilization at different scales, and their validity should be assessed across soil layers.