Modified rothamsted carbon model for andosols and its validation: changing humus decomposition rate constant with pyrophosphate-extractable Al

We modified the Rothamsted carbon model (RothC) to adopt it to Andosols. We changed the decomposition rate constant of the humus (HUM) pool because the presence of Al-humus complexes in Andosols gives HUM strong stability. In addition, the inert organic matter (IOM) pool was set at zero, because the soil did not contain carbon when it was formed from fresh volcanic ash. H(f) was defined as the factor required to divide the decomposition rate constant of the HUM pool so that the modelled soil organic carbon (SOC) level matched the measured level. H(f) was calculated for 32 Japanese Andosols, and the relationships between H(f) and soil properties such as contents of acid-oxalate-extractable Al, Fe, and Si (Mo, Feo, and Sio) or pyrophosphate-extractable Al (Alp) were analysed. The equation H(f) =1.20 + 2,50Alp(% ), (r 2 = 0.518) was selected to divide the decomposition rate constant of HUM. The modified model was tested for four long-term experimental data sets on Andosols under various climatic conditions, soil textures, and management techniques from north to south across Japan. We obtained considerably improved fits between the contents of modelled and measured SOC by using the modified RothC instead of the original model. Use of this modified model may contribute to the improvement of the performance of existing SOC2 models in modelling Andosols. It can be used for estimating CO2 emission from soils, as well as for planning suitable organic matter management in Japanese Andosols.