Land use selectively impacts soil carbon storage in particulate, water-extractable, and mineral-associated forms across pedogenetic horizons

Improved understanding of land use derived changes in soil organic matter (OM) compartments stabilized to different degrees against microbial decomposition is required for outlining efficient land use strategies aimed at improving soil ecosystem functions that are strongly coupled to gains and losses of soil organic carbon (OC). However, such data is scarce, particularly in subsoil environments. Consequently, in this study, we analyzed OC storage in topsoils and subsoils, as well as OM fractions with different OC turnover dynamics, including particulate (free and occluded), water-extractable, and mineral-associated OM. We sampled soils under native prairie (10 sites) and long-term arable use (> 40 years, 10 sites) to a depth of 3 m in the central U.S. Our results showed that the arable bulk soils had significantly lower OC content in the A horizon and across all analyzed OM fractions compared to native prairie soils. This reduction was primarily derived from OC losses in the mineral-associated OM (arable: 7.2 ± 0.5 g kg⁻¹; native prairie: 12 ± 0.7 g kg⁻¹), which retained the most significant portion (50–56 %) of bulk soil OC among all fractions. No significant impact of land use on OC storage in the bulk soil and fractions was observed in the subsoil B and C horizons, except for water-extractable OM, which had lower amounts in arable soils in the C horizon than native prairie soils. This underscores the relevance of this fraction for the translocation of OC across the soil profile in undisturbed systems. Our results highlight the crucial role of mineral-associated OM for soil OC storage, but also its sensitivity to land use change, especially in the topsoil, suggesting this fraction is highly relevant for strategies aiming at restoring pre-disturbance soil OC levels.