Quantitative relationship between organic carbon and geochemical properties in tropical surface and subsurface soils

We investigated soil organic carbon (SOC) storage and the quantitative relationship between SOC and geochemical properties in tropical surface and subsurface soils, where information on various soil geochemical properties remains limited. We used soil samples from 178 sites in Indonesia, Thailand, Cameroon, and Tanzania as tropical soils and 34 sites in Japan as a comparison. The tropical soils were acidic, formed under high precipitation. We separated the soil samples into three groups based on the degree of weathering. We applied correlation and single regression analyses between SOC and geochemical properties including oxalate extractable Al and Fe (Alo + Feo), pyrophosphate extractable Al and Fe (Alp + Fep), and clay + silt. In the subsurface soils, Alo + Feo and Alp + Fep were strongly correlated with SOC (r = 0.79–0.85 and 0.58–0.85, respectively), and the quantitative relationships of SOC with these properties were similar in all the weathering groups (4–5 and 5–8, respectively). On the other hand, no strong correlation with SOC or similar quantitative relationships among the weathering groups were found for clay + silt. In the surface soils, Alo + Feo and Alp + Fep were correlated with SOC (r = 0.48–0.56 and 0.55–0.76, respectively), but the ratio of C to active Al and Fe was higher probably because of the large C input. Our results highlight the importance of Alo + Feo and Alp + Fep in SOC storage, of which the ratios to SOC are similar in each of the surface and subsurface soils regardless of the weathering degree.