Impact of solar radiation during flowering stage and precipitation on soybean yields in northern Kyushu, Japan

Soybean production in northern Kyushu (Saga and Fukuoka Prefectures), Japan, has declined since 2008. As major cultivar and cultivation management have hardly changed in recent years, this decrease should be caused by climate variability. Thus, we evaluated the variability of the recent climate in northern Kyushu and its impact on soybean production. Trend analyses and the Lepage tests were conducted on air temperature, solar radiation, and precipitation over the past 30 years. The correlations between these meteorological conditions and soybean yield were assessed. Solar radiation from late August to early September, corresponding to the soybean flowering stage, declined since 2008 and showed a strong positive correlation with yield, suggesting a critical limiting factor. Shifting the flowering stage through early or late sowing could mitigate this effect. Precipitation during the growing season was negatively correlated with soybean yield, indicating that improvements in drainage management are effective. Interestingly, soybean yields remained high under low precipitation conditions, showing that soil drought did not significantly limit production. These findings highlight the need for adaptive strategies, including adjustments of sowing date and improved water management, to enhance soybean production in northern Kyushu.