Assessing changes in soil carbon stocks after land use conversion from forest land to agricultural land in Japan

•Soil C stocks in agricultural land after deforestation were assessed in Japan.•In Andosols, soil bulk densities in forest land were lower than those in cropland.•It is therefore important to use an equivalent soil mass approach for Andosols.•Most of cropland and grassland lost soil C after deforestation in Japan.•Mean annual air temperatures at the sites impacted changes in soil C stocks.