Soil organic carbon pools controlled by climate and geochemistry in tropical volcanic regions.

Understanding the factors that control the storage of soil organic carbon (SOC) is an urgent priority for mitigating global climate problems. The objective of this study was to determine the factors controlling SOC pools with differing stabilities. Surface soil samples were collected along an elevation gradient from four volcanic regions of Tanzania (two regions) and Indonesia (two regions) under largely-undisturbed vegetation (24 sites in total). A three-pool kinetic model was fitted to accumulative CO release curve produced over 343-day incubation to determine the sizes of the labile and intermediate SOC pools (C and C, respectively) and their mean residence times (1/K and 1/K, respectively), where the size of the stable SOC pool (C) was measured as non-hydrolyzable carbon. Correlation and path analyses were performed using the results of soil fractionation and model fitting with climatic and geochemical properties. The intermediate pool comprised 50% of total SOC, was responsible for 58% of total accumulative CO release, and controlled total SOC stability. The content of nanocrystalline minerals (Al + 1/2Fe: 5.5-110 g kg) was strongly correlated with C and C, suggesting that organo-mineral complexes is the essential factor that controls C and C rather than soil texture or pH. Temperature (12-26 °C) was weakly correlated with C, C, and strongly with C which was closely related to microbial biomass carbon. The low temperature at the high elevation sites retards the decomposition of the whole SOC. The significant correlations of excess precipitation with 1/K and 1/K represent the effect of moisture on the potential stabilities of the labile and intermediate SOC pools. Climatic factors primarily affect relatively labile SOC pools, whereas geochemical factors influence more stable pools and control total SOC. The findings have important implications for understanding the SOC stabilization mechanisms, which is an essential process of the carbon cycle, in tropical volcanic soils.