Factors controlling accumulation and decomposition of organic carbon in humus horizons of Andosols: A case study for distinctive non-allophanic Andosols in northeastern Japan

Andosols often accumulate soil organic matter (SOM) in large amounts. To investigate the factors controlling the stability and lability of organic carbon (OC) in humus horizons of Andosols, we selected 19 A horizon samples (surface and subsurface horizons) from the Field Station of Tohoku University including areas where benchmark soil profiles of non-allophanic Andosols are distributed. We determined the soil properties possibly controlling the OC accumulation, such as pH(H₂O), 1 M KCl-extractable aluminum (KCl-Al), pyrophosphate-extractable Al and iron (Al_p, Fe_p), acid oxalate-extractable silicon (Si_o), total OC, water-extractable OC, and humified OC. To evaluate the OC mineralization, we measured the soil respiration rates in a laboratory for non-treated, neutralized (CaCO₃, Ca(OH)₂ and NaOH), and nutrient applied (KH₂PO₄, (NH₄)₂SO₄) soil samples. Statistical analyses, including a path analysis, showed that the Al_p and pH(H₂O) values are directly related to the OC concentration (P < 0.01 and P < 0.05, respectively). There was a significant negative correlation (P < 0.01) between the soil respiration rates of the non-treated samples and the ratios of the humified OC to total OC, showing that the humification of the SOM was definitely related to the OC stability. Effects of the chemical treatments to the soil respiration rates were greater in the surface horizon samples with an abundant labile OC than those in the subsurface samples. Neutralization affected the soil respiration rates more significantly than the nutrient application. Among the neutralization treatments, the liming materials more effectively increased the respiration rates. This was probably due to an increase in the lability of the humified OC by liming.