Soils Affected by Conversion of Abandoned Paddy Fields to Alternate Uses

In Japan, agricultural land use has constantly decreased since 1961, when the cultivated area reached 6 million ha. Here, the research area formerly used as paddy fields has been replaced by sections for buckwheat cultivation, chestnut orchards, and a parking lot. The region located at higher elevation in the valley plains study area was converted to upland fields by drainage and soil dressing in 1975, and was followed by conversion to a chestnut orchard in 1990. Both soils were Skeltic Fulvisols. The middle elevation area was converted to a parking lot by paving with 2 cm thick asphalt, thus the soil is classified as Ekranic Technosol. The lower elevation area has been transformed into a buckwheat field in 2000. Soil texture varies widely depending on distribution of alluvium in the soil profile, indicating diverse intensity of stream flow and alluvial deposition processes. Construction of a hard pan layer was inevitable to manage water in the paddy field. The hard pan layer with finely textured soil was confirmed in the chestnut orchard and in the parking lot. The distribution of oxide mottles in the soil profile under the chestnut orchard and the parking lot was proof of the past use of the field. Values of pH of all surface soils at the three soil profile sites were slightly acidic. Land use conversion from the paddy field has not influenced soil reactions. The difference in bulk density at each site can be indicative of land use conversion. Compaction beneath the asphalt promotes high bulk density, while low bulk densities were brought about by development of chestnut root systems.