Yield loss of oilseed rape (Brassica napus L.) under nitrogen deficiency is associated with under-regulation of plant population density

Yield loss in oilseed rape is caused by a decline in plant population density (PPD) due to nitrogen (N) insufficiency. The optimization of PPD is the foundation of the high and stable yield of winter oilseed rape. Field experiments were conducted with five contrasting N applications (0, 60, 120, 180 and 240 kg N ha(-1), denoted by NO, N60, N120, N180 and N240, respectively) to identify the effect of different N application rates on PPD dynamics and the influence of PPD at harvest (PPDh) on seed yield. Oilseed rape yield was increased by the increase of PPDh and yield per plant as the N application rate added. The N deficiency decreased PPDh, resulting in a 35.1% and 17.1% yield loss in the NO and N60 treatments compared to the PPDh of the treatment with the highest yield. This loss accounted for 30.9% and 20.6% of the total yield loss in the both treatments, respectively. PPD declined after the seedling emergence over the whole growing season, and the N application rate exhibited different effects at different growth stages. Sufficient N supplying suffered a sharp decline in PPD mainly during the seedling stage, accounting for approximately 46.1% similar to 48.1% of the total mortality. As the N supply was insufficient, PPD declined dramatically both at the seedling stage and after the flowering stage, with the mortality proportion of 38.2% and 39.7% in the NO treatment, respectively, resulting in greater mortality than the other stage over the growth period. A significant difference in the degree of PPD decline between sufficient and insufficient N treatments was observed since 150 days after sowing (DAS) till the harvest stage. Reduction in PPD was regulated by plant N status, and there was a negative relationship between N concentration and PPD. The minimum N concentrations of minimum decline in PPD from seedling emergency to 60 DAS and from 150 DAS to harvest were 4.11% and 2.44%, respectively. A lack of N increased plant mortality led to the reduced PPDh and decreased yield. PPDh should be optimized by the reasonable N management to promote individual growth, guarantee the group development, and achieve economic and efficient production.