Factors affecting organic carbon accumulation in humus horizons of volcanic soils from the Tohoku University World Andosol Database:

A path analysis

Abstract

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 (Al_p and Fe_p, humus complexed Al and Fe), and acid oxalate-extractable Si (Si_o, 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 Al_p (r = 0.69, P < 0.01) was mainly explained by the direct effect of Al_p (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 Al_p. 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.