This study was conducted to determine the relationship between the soil organic carbon (OC) content and selected soil properties such as pH(H2O), 1 M KCl-extractable aluminum (KCl-Al, exchangeable Al), pyrophosphate-extractable Al and Fe (Alp and Fep, humus complexed Al and Fe), and acid oxalate-extractable Si (Sio, Si in allophanic minerals) in 293 A horizons from the Tohoku University World Andosol Database. A path analysis was used to examine the direct effects and indirect effects of the soil properties on the OC content. The path analysis showed that a high correlative coefficient between the OC content and Alp (r = 0.69, P < 0.01) was mainly explained by the direct effect of Alp (0.52, P < 0.01) on the OC content. Strong correlations between OC and KCl-Al (r = 0.60, P < 0.01) or pH(H2O) (r = -0.58, P < 0.01) were due not only to the direct effect (0.21 and -0.27, respectively, P < 0.01), but also to the indirect effects of other properties, especially that of Alp. Thus, it is considered that, in the humus horizons of the Andosols, the Al-humus complexation mainly contributes to the OC accumulation, and the low soil pH values and Al toxicity partly relate to the humus reservation by depressing the microbial activity.
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