A continental-scale study of Spodosols across North America and implications for soil organic carbon dynamics

Spodosols are among the most prevalent soil orders in temperate forests and play a key role in carbon sequestration and climate regulation. Podzolization promotes the accumulation and mobilization of mineral-associated organic carbon (MAOC) in deeper horizons, and the concentration of Fe Al oxyhydroxides controls the efficiency of this process and the carbon storage capacity of these soils. Yet, no large-scale study has compiled Spodosol data to evaluate soil properties, podzolization thresholds, and the influence of Fe Al oxyhydroxides and clays on MAOC storage. Here, we present a continental-scale compilation of over 3000 Spodosol profiles from across North America to assess SOC, Fe Al oxyhydroxides, clay content, and development thresholds based on parent material Fe content. We also estimated carbon saturation and maximum MAOC storage capacity using both silt + clay and Fe Al oxyhydroxide concentrations. Our results show that coastal regions exhibit exceptionally high SOC and Fe oxyhydroxide levels, while more continental regions have lower concentrations. Parent material Fe content was significantly higher in coastal regions, especially Coastal Alaska (12 %), compared to <4 % in continental areas. We demonstrate that concentration of Al and Fe oxyhydroxides is a more reliable approach of estimating MAOC storage capacity than silt + clay content. The mineralogical capacity of Spodosols, based on Fe Al oxyhydroxides, supports maximum MAOC concentrations ranging from ∼2 % in continental regions to nearly 10 % in Coastal Alaska. These findings highlight the role of climate in driving podzolization intensity and show that Spodosols, especially in northern coastal regions, hold significant potential for additional carbon sequestration.. • Coastal Spodosols store more SOC and Fe Al than continental counterparts. • Fe thresholds for podzolization vary widely with climate and parent material. • Fe + Al explain C saturation and storage capacity better than silt + clay in Spodosols.