Abstract: In this study, potted experiments were conducted using the early-maturing soybean variety "Heihe 43" and the late-maturing soybean variety "Dongnong 53" under low temperature treatment (LT). The shoot and root dry weights, nodule biomass, total nitrogen content, and nodule nitrogen fixation were measured in the 28-49 days after soybean emergence. The results showed that: (1) low-temperature stress encountered by soybeans in the first week after emergence would delay soybean growth and development. From the 28d to 42d after emergence, soybean biomass was significantly lower than that of the CK treatment under low-temperature stress, but no significant difference was observed in the biomass of the two soybean varieties until the 49d after emergence. (2)In comparison to CK, significant inhibition of nodule formation in "Dongnong 53" and "Heihe 43" by low temperature occurred in the 42d and 35d after emergence, and the number of nodules decreased by 58.8% and 72.0%, respectively. During the 49d after emergence, low temperature decreased the dry weight of nodules of "Dongnong 53" and "Heihe 43" by 48.9% and 48.5%, respectively. (3) Low temperature stimulated the nitrogen fixation ability of two soybean varieties. At 49d after seedling emergence, the biological nitrogen fixation, nitrogen fixation per unit nodule biomass and nitrogen fixation per individual nodule of "Dongnong 53" and "Heihe 43" at low temperature increased by 89.9%, 118.9%; 249.3%, 172.6% and 150.6%, 114.2%, respectively. The percentages of biological nitrogen fixation increased by 26.4 and 24.5 percentage points, respectively. Collectively, low temperature stress after soybean emergence can slow down growth and development, and inhibit the formation and growth of nodules. However, soybean growth will gradually recover after the release of low temperature stress. Short-term low temperature stress has a stimulating effect on the nitrogen fixation of soybean nodules and their nitrogen fixation ability, and the stimulating effect is more pronounced for the early-maturing variety "Heihe 43". Therefore, it is recommended choosing early-maturing soybean varieties to better adapt the early spring low temperature environment in the soybean production area of Northeast China.

Key words: Early and late maturity soybean, Low temperature stress, Nodule, 15N natural abundance method, Nodule nitrogen fixation