Factors controlling sizes and stabilities of subsoil organic carbon pools in tropical volcanic soils

Soil organic carbon (SOC) in the subsoil may not be so resistant to decomposition as previously assumed, while the mechanisms controlling C dynamics in subsoils are not yet known. This study aimed to (1) identify the factors that control SOC pools in subsoil and (2) compare the differences in SOC pools and controlling factors between the subsoil and topsoil. Subsoils (20-40 cm) were sampled along elevational gradients from two volcanic regions with less-disturbed vegetation each from Tanzania (11 sites) and Indonesia (12 sites). The sizes and mean residence times of labile, intermediate, and stable SOC pools were estimated by fractionation and model fitting to CO₂ release during long-term incubation. The controlling factors of each SOC pool were determined by accompanying partial correlation and path analyses. In subsoil, the intermediate SOC pool predominantly controlled the SOC stability within decades. Climatic, geochemical, and biotic factors controlled different SOC pools. Temperature negatively affected the sizes of all three pools. The nanocrystalline minerals contents predominantly and positively controlled the sizes of intermediate and stable SOC pools, and the mean residence time of intermediate SOC pool. Biotic and climatic factors (i.e., microbial biomass, available N for microbes, and excess precipitation) controlled the labile SOC pool. Compared with topsoil, stabilized organic matters were more in the intermediate rather than in the stable SOC pool, and the temperature had a more significant effect on the stable SOC pool in subsoil than in topsoil. Available N for microbes partially controlled the labile and intermediate SOC pools in subsoil (more limited available N for microbes), but not in topsoil. Thus, subsoil SOC would be more sensitive to climate change than topsoil SOC. This study helped to understand the SOC stabilization mechanism and emphasized the high climate- and mineral-dependence of SOC in subsoil of tropical volcanic regions.