When do rice terraces become rice terraces?

Rice terraces located on sloped hills in mountainous areas are sometimes abandoned because of lack of labor and difficulties associated with cultivation. Recently, rice terraces also have been re-evaluated from the perspectives of water conservation or flood control. As a consequence, restoration has been conducted in many locations. In this study, we conducted field research on rice terraces in Okayama in order to determine when rice terraces become stable as rice terraces. Soil samples were obtained from depths of 0, 10, and 30 cm in abandoned fields and rice terraces restored in 2015, 2016, and 2017. The results showed that total carbon was the highest in abandoned fields and gradually decreased in the 2017-restored fields, followed by that in the 2016-restored fields. Particle analysis, particularly of surface soil, showed that the sand fraction decreased in order of abandoned fields, 2017-restored fields, and 2016-restored fields. We established that recently restored rice terraces have similar properties to those located on mountain hills, including few fine particles and high organic matter content in the surface soils. If the surface soil was cultivated and mixed with deeper soils, then the clay and silt content would gradually increase with time. Cone penetrometer analysis revealed that there was no clear evidence of a hard pan in the abandoned field and the 2017-restored field. In contrast, a hard pan had begun to develop in the 2016-restored fields and that it had become well established in the 2015-restored fields. Collectively, our findings indicate that although all rice terraces have a superficially similar appearance, recently recovered fields tend to have properties comparable to the properties of those located on mountain hills. These properties gradually change in the 2–3 years following restoration with the subsequent development of a well-established hard pan. We conclude that even rice terraces that have been abandoned for more than 30 years can develop clear properties of rice terraces within 3 years after restoration.