Objectives To investigate the effects of nitrogen application rates on the yield of winter rapeseed in normal and freeze years, clarify the relationship between key yield-reducing traits and N application rates, and provide nutrient management to mitigate the freeze damage on rapeseed production.

Methods Field experiments were conducted in Hubei Province, the middle reaches of the Yangtze River, during the 2022/2023 and 2023/2024 growing seasons. Five N fertilizer rate gradients (0, 90, 180, 270, and 360 kg/hm², denoted as N₀, N₉₀, N₁₈₀, N₂₇₀, and N₃₆₀) were set. The days of growth stages in both years were investigated, and the corresponding heat indices were calculated. At maturity, indices including aboveground dry matter weight and distribution ratio, yield and its components, and rapeseed particle size were measured. The freeze damage degree of winter rapeseed under different N treatments was evaluated by principal component analysis and membership function method.

Results In February 2024, severe low-temperature freeze damage occurred during the bud and bolting stage of winter rapeseed in Hubei Province, designating this year as the freeze damage year, while 2023 was the normal climate year. Compared with normal years, each nitrogen fertilizer treatment in the 2023/24 season extended the budding and stem elongation stage by approximately 7 days, resulting in a shortening of the flowering stage by 3−9 days and the silique-maturity stage by 1−4 days, respectively. Compared with N₀, other N treatments increased rapeseed yield by 947.0−1588.2 kg/hm² in 2022/2023 and 615.3−1334.8 kg/hm² in 2023/2024, with yield increase rates of 101.66%−170.50% and 179.91%−390.29%, respectively. The freeze damage caused yield reduction in N₀ exceeded 60%, whereas that in N₉₀, N₁₈₀, N₂₇₀, and N₃₆₀ treatments were declined to 49.0%, 33.5%, 36.6%, and 40.7%, respectively. The minimum decrease of N use efficiency caused by freeze damage was also observed at N₁₈₀, with the partial factor productivity of N and agronomic efficiency of N₁₈₀ decreased by 33.46% and 15.96%, while the decrease at N₃₆₀ were by 40.70% and 39.34%, respectively. The impacts of freeze damage on key traits at maturity varied with N rates. Compared to normal year, N₀ treatment in freeze year were observed significant decrease in plant height, number of branches, seeds per pod, and 1000-seed weight, N₁₈₀ were observed smaller reductions in these traits, while N₃₆₀ exhibited more serious damages in traits, with an average reduction of 143 pods per plant and a 41.53% decrease in branch pods, and freeze stress also reduced the overall seed size by 0.2 mm in N₃₆₀. Comprehensive analysis revealed the key trends for evaluating freeze damage at maturity were decreased aboveground dry matter weight, increased stem dry matter distribution ratio, reduced 1000-seed weight, and decreased number of pods per plant.

Conclusions In the middle and lower reaches of the Yangtze River, freeze damages the aboveground dry matter accumulation and their allocation to seeds, so reduces pods number and 1000-seeds-weight, and the yield as a consequent, low or excessive nitrogen application will exaggerate the damage. Enhancing nitrogen application rate appropriately will effectively alleviate the loss of yield and fertilizer efficiency. The proper rate is N 180 kg/hm² in normal climate years, and should be increased to 228 kg/hm² in freeze years, while exceeding 228 kg/hm² will exacerbate the damage.