Change of soybean climatic suitability in Northeast China under climate change

Northeast China is a major soybean production region that is profoundly influenced by climate change. It is important to identify how climate change influences agroclimatic resources, agro-meteorological disasters, and climatic suitability for soybean production and plant expansion. In this study, we evaluated the temporal and spatial changes in agroclimatic resources (accumulated temperature, precipitation, and sunshine hours at a rate of 80% climatic guarantee during soybean growing season), agro-meteorological disasters (drought and frost days), and soybean climatic suitability using two climate reference periods (1991−2020 and 1981−2010). Finally, the contribution rates of accumulated temperature, precipitation, and sunshine hours to soybean climatic suitability change were calculated using a statistical method. First, we found that a thermal time ≥ 10 ℃ during the soybean growing season increased by 26−65 ℃·d in the east of Inner Mongolia, southwest and northeast of Heilongjiang, and midwest of Jilin, and increased by 66−182 ℃·d in local regions. Additionally, a thermal time ≥ 10 ℃ during the soybean growing season exhibited an increasing trend from 1981 to 2020 in Northeast China. Moreover, the changes in precipitation and sunshine hours during the soybean growing season had the characteristics of spatial differentiation. Second, the number of frost days during the soybean growing season had decreased in the major region of Northeast China by 1–3 days and by 4–6 days in some local areas. Drought days declined in Heilongjiang and northwest of Jilin by 1–4 days and increased in mideast of Jilin and Liaoning by 1–6 days. Third, the climatic suitability of soybean planting has improved in the eastern Inner Mongolia, west of Heilongjiang and Jilin, and Liaoning, while it had dropped in the middle and east of Heilongjiang and the middle and northeast of Jilin. Finally, the change in sunshine hours was the foremost factor affecting soybean climatic suitability. The secondary factor was thermal time, whereas precipitation had the least contribution. The results indicate that the climatic conditions of soybean in 1991–2020 in Northeast China were more favorable than those in 1981–2010. Farmers in Northeast China adopted the sowing date and soybean cultivar to fully utilize the temperature resource. However, blindly growing soybeans across the accumulated temperate zone was not encouraged because of the increased extent of thermal time.