Impact of farmland reclamation on soil distribution in Japan: the case of Andosols in Nagano Prefecture

In modern times, farmlands in Japan have been reconstructed at a large-scale through the cutting and filling of soils. In principle, such reconstruction should be carried out with a minimum amount of soil transportation to decrease environmental impacts and cost, and the soil distribution and properties should be determined by the subsequent reclamation process. To assess this issue, we attempted to evaluate the impacts of farmland reclamation on soil distributions in Japan. Farmland soil in the reconstructed area was surveyed from a pedological perspective using geographic information system (GIS) data. To enable a simple comparison of topsoil thickness and soil distribution before and after farmland reclamation, we selected survey soils classified as “High-humic Cumulic Allophanic Andosols” under the Japanese soil classification system, which are more than 50 cm in thickness in the Melanic horizon. Changes in the thickness of the topsoil were evaluated by direct measurement at 30 field survey positions. The extent of farmland affected by large-scale reclamation was calculated from a GIS-based comparison of topography before and after reclamation based on a digital elevation model (DEM). The reclamation process decreased the thickness of topsoils from more than 50 cm to an average of 26.9 cm over the 30 surveyed positions. The minimum topsoil thickness of 15 cm observed in this study corresponds to the recommended topsoil conservation level under Japanese guidelines for the reclamation of paddies. There was no significant difference in the topsoil thicknesses obtained through soil filling and cutting reclamation processes. These results indicate that the original soil distributions potentially lost the “High-humic Cumulic” feature. Although the topsoils were conserved during farmland reclamation processes carried out in accordance with the Japanese guidelines, the results of this study revealed the influence of the reclamation process on soil properties and, as a result, their classification, through a decrease in topsoil thickness and mixing with subsurface horizons. We confirm the importance of taking topsoil conservation into account in determining the influence of agricultural reclamation on soil distribution in Japan. In reclaimed upland fields, most managed topsoils have sufficient thickness to fill the diagnostic surface horizon. An exception is managed topsoil in reclaimed paddies, which can have insufficient to thickness to meet the corresponding diagnostic surface horizon. We suggest that some soils in modern reclaimed farmlands do not currently fulfill the criterion of diagnostic surface horizon regardless of their reclamation magnitude.