Temporal trends in soil types in paddy fields across 12 prefectures in Japan

<p>The 1/50,000 cultivated soil map of Japan is based on the Fundamental Soil Survey for Soil Fertility Conservation conducted during 1959–1978; it does not reflect present land-use types or recent soil survey results. Recent progress toward reformation into well-drained paddy fields may have lowered the position of the groundwater gley horizon; therefore, the distribution area of Gley Lowland soils, one of the major soil types in paddy fields in Japan, may have decreased. In this study, we conducted simple soil profile surveys at 1,474 sites in paddy fields across 12 prefectures in Japan (Hokkaido, Aomori, Iwate, Akita, Ibaraki, Chiba, Niigata, Aichi, Shiga, Hyogo, Nagasaki, and Kagoshima) and analyzed temporal differences in soil types by comparing the soil map with the present survey results, especially focusing on land-use and landform types. We found that the proportion of Gley Lowland soils had decreased from 76.4％ to 43.8％, whereas that of Gray Lowland soils had increased from 9.1％ to 32.0％. In addition, the proportion of soil groups in which the groundwater gley horizon appeared within 50 cm of the soil surface tended to have decreased to a greater extent in paddy–upland rotations or converted upland sites relative to in continuous paddy sites. Comparison by landform revealed a wide variety of trends in soil-type differences, but specific trends, similar to those detected for land use, were not found. Thus, the nationwide reformation into well-drained paddy fields has apparently lowered the position of the groundwater gley horizon, changing Gley Lowland soils into other soil types, such as Gray Lowland soils. We propose that the present land-use type is an important factor for determining the degree of soil-type change from Gley Lowland soils to other soil types in paddy fields.</p>