Distribution and Characteristics of Acid Sulfate Soils as a Guide in the Restoration of Paddy Fields Affected with Severe Cracks by the 2016 Kumamoto Earthquake
The 2016 Kumamoto earthquake caused large ground cracks to appear in the paddy fields in Aso Valley within the Aso caldera in Kumamoto prefecture. The local government planned to restore the cracked paddy fields but was aware that acid sulfate conditions（soils with pH（H2O2）<3.0 and sulfate ions in H2O2-treated soil extract）occurred in the lower soil layers in some parts of the Aso valley. Acid sulfate soils release sulfate ions that form sulfuric acid when the soils are subjected to aeration and oxidation, so it was important that acid sulfate soil layers not be exposed during the restoration process. Therefore, the distribution of acid sulfate soils in Restoration Areas 1 and 2（five fields per restoration area）was mapped, and the chemical and visible characteristics of the soils were determined for information and guidance in the restoration process.
In Restoration Area 1, acid sulfate soils existed in shallow soil layers and were highly acidic
（pH（H2O2）< 1）. In Restoration Area 2, acid sulfate soils existed only at depths below 80 cm and were weakly acidic（pH（H2O2）~3）. The pH（H2O2）values of the acid sulfate soil samples measured one week after sampling were highly correlated with the pH（H2O）of the same soils after being air-dried for approximately three months, confirming that pH（H2O2）of fresh samples is useful for assessing the status of acid sulfate soil. The acid sulfate soils were olive black to black in color, emitted hydrogen sulfide smell and contained discernible partly decomposed plant materials with a peat-like appearance. The information obtained was used by the local government office of Kumamoto prefecture as a guide in the restoration of the paddy fields affected by the earthquake
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熊本地震により被災した阿蘇谷の水田における酸性硫酸塩土壌の分布状況とその特徴
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