Objectives The autumn waterlogging is one of the common constraint factors in winter rapeseed production. In this research, a field experiment was carried out to study effects of water logging at the seedling stage and nitrogen (N) application on seed yields and N use efficiency of direct-sown winter rapeseed, to provide a theoretical foundation for reasonable utilization of N fertilizer in direct-sown winter rapeseed under the waterlogging stress.

Methods The field experiment was conducted by using a local major rapeseed cultivar, Huayouza 9. Direct-sowing was the main cultivation pattern. A split-plot experiment was designed, and the main plots were soil water conditions which consisted drainage treatment and waterlogging treatment at the seeding stage and N application rates were sub-plots at six levels, 0, 60, 120, 180, 240 and 300 kg/hm². Growth characteristics, dry matter and N accumulation, and N utilization were studied at the seedling, budding, flowering and maturity stage, meanwhile, the yield and its components were investigated.

Results The growth of rapeseed and yield were obviously inhibited by waterlodging at seedling stage. Compared with the drainage, waterlogging significantly reduced the number, area and the SPAD values of leaves at different degrees under the same N levels. The dry matter amounts were reduced by 19.1%-26.5%, and the harvest indices were decreased by 5.2 percentage points after the waterlogging. The seed yield was significantly decreased by 23.6% under the waterlogging stress, and the decreased seed yield resulted from a decrease in density at the five leaf stage and the number of siliques per plant, which were reduced by 7.6% and 20.4%, respectively. Moreover, the N content of the plants was decreased by 8.0 percentage points, the N accumulation amounts were reduced by 23.2%-32.4%, and the N partial factor productivity (PFP_N), N agronomy efficiency (AE_N) and N recovery efficiency (RE_N) were declined by 22.8%, 20.4% and 18.6% under the waterlogging stress, respectively. Suitable nitrogen application could alleviate the effect of the waterlogging on the growth and yield of winter rapeseed, and there was a direct relationship between the responses of the yield to the waterlogging stress and N input. The N application significantly enhanced leaf number, leaf area and the SPAD value of leaves, increased density, N content, population quality and the yield, reduced the RIR (relative damage percent) of rapeseed yield which declined from 35.3% to 13.8% with the increment of N application. The decreases of the dry matter and N accumulation amounts were alleviated by the N application under high N levels range from 240 to 300 kg/hm² and decreased by 9.5% and 15.0%, respectively, while they were decreased by 27.6% and 32.9% under the middle or low N levels range from 0 to 180 kg/hm². Therefore, under a certain amount of N fertilization, increasing the N rate could show compensation effect on direct-sown winter rapeseed growth and recovery. Compared with the waterlogging treatment, 59.2 kg/hm² of N fertilizer was reduced in the drainage treatment.

Conclusions The inhibiting effect of the waterlogging on rapeseed not only affected the seedling stage, but also impacted other growth stages, and eventually led to the yield loss. The N application could increase the grain yield to alleviate the negative effects by optimize population quality for rapeseed under the waterlogging stress. So, suitable N fertilization rate and real-time drainage would be feasible and realizable with obtaining on both waterlogging resistance by N rate and N reduction by drainage.