Historical impacts of cultivated land changes on national and regional soybean yields in Japan based on official statistics (1979–2023)

In Japan, the national average soybean yield rose to 190 g m−2 (1.9 t ha−1) by 2000 but has recently declined to 160 g m−2. This study quantifies the impacts of cultivation area movements (CAM) and paddy-converted field use (CFU) on national and regional average soybean yields using official statistics. We constructed panel data on yields and cultivated areas for converted and upland fields in 49 prefectures/districts from 1979–2023 to estimate average yield changes attributable to CAM and CFU. The CFU-attributable national yield gain increased to 15 g m−2 in the 1980s and contributed to yield rises by 2000. The gain persisted through the 2020s, indicating that CFU was not a major factor responsible for post-2000 yield declines, in contrast to earlier agronomic interpretations. The CAM-attributable national yield gain increased gradually to 18 g m−2 from 2000 because of shifts to high-yielding Hokkaido, thereby mitigating these declines. Additional regression analyses indicated that post-2000 yield declines were driven by factors common to both field types. Overall, recent national soybean yields in Japan have increased by more than 20% due to CAM and CFU, demonstrating the positive impacts of cultivated land changes and providing insights into interpreting yield changes in official statistics.