Effects of nitrogen fertilizer application on oilseed rape yield in upland and paddy fields and the extent of yield reduction due to freezing damage

Objectives Low temperature freezing damage is a common limiting factor in winter rapeseed production. This experiment explored the differences in nitrogen fertilizer application and frost damage response of winter rapeseed under different planting systems, in order to improve the efficiency of nitrogen fertilization on alleviating freezing stress to oilseed rape.

Methods A localized field fertilization experiment was started under maize-rapeseed and rice-rapeseed rotation systems in 2012, and four N application rates were set up: 0, 75, 150, and 225 kg/hm² under each of the two rotation systems. We investigated the climatic conditions during the rapeseed growth seasons of 2022/2023 and 2023/2024, as well as the growth status during the overwintering and bolting periods of rapeseed. At the harvesting period, we investigated the biomass, yield, and yield components of rapeseed.

Results During the overwintering period of rapeseed in 2023/2024, the incidence and duration of freezing damage were notably severer compared to the previous season, 2022/2023, classifying 2023/2024 as a year of significant freezing stress. Compared with normal year 2022/2023, freezing stress in upland and paddy field decreased the average number of pods per plant by 36.5% and 33.7%, biomass by 54.9% and 50.4%, and yield by 49.2% and 44.2%, respectively. Throughout the overwintering and bolting stages, upland rapeseed experienced severer frost damage than paddy rapeseed. Irrespective of freezing stress, increasing nitrogen fertilizer application significantly boosted the number of siliques per plant, enhanced biomass and harvest index, and had a more pronounced yield-increasing effect on upland rapeseed. Compared with normal year 2022/2023, under 2023/2024 freezing stress, the average yield of rapeseed in upland and paddy fields decreased by 49.8% and 45.1%, and the number of pods per plant decreased by 37.0% and 34.5%, respectively. With the increase of nitrogen application rate, the magnitude of yield reduction decreased. Notably, when the nitrogen application rate reached 225 kg/hm², both upland and paddy rapeseed achieved peak yields and related agronomic indices.

Conclusions Freezing stress during the overwintering and bolting stages significantly reduced the yield of rapeseed in both upland and paddy fields, with upland rapeseed experiencing even greater losses compared to paddy field rapeseed. Applying optimal amount of nitrogen fertilizer can effectively mitigate the freezing damage by boosting the number of siliques, facilitating the transfer of photosynthetic products to seeds, and improving the harvest index. Furthermore, the beneficial impact of nitrogen fertilization is more pronounced in upland fields than in paddy fields. Regardless of freezing stress, the maximum yield response for rapeseed in both cropping systems was observed at N application rate of 225 kg/hm². Consequently, it is advisable to adopt this appropriate nitrogen application rate locally.