Suitable nitrogen fertilization mode for high yield and quality and high N use efficiency in high density direct-sown rapeseed(Brassica napus L.)

【Objectives】 The reasonable nitrogen fertilizer management strategies in rapeseed(Brassica napus L.) with high planting density(36×104 plant/hm2)and direct-sown conditions were studied. 【Methods】 Two field experiments were conducted in Chengdu Plain under a rice-rapeseed rotation system. The effects of different N rates(0, 90, 180, 270, 360 kg/hm2) in the same N application mode(a basal fertilizer and a top dressing) and different N application modes(single application of basal fertilizer, twice application of a basal fertilizer and a top dressing, three times of application as a basal fertilizer and two top dressings) in the same N rate(225 kg/hm2) on yield, quality and nitrogen fertilizer use efficiency in rapeseed were studied.【Results】 The results indicated that the increase of N application rates demonstrated an enhancement trend in agronomic traits in direct-sown rapeseed. However, the effects of N application modes on plant height, branch number and pods per plant were not significant. With the increase in N application rates,the seed yield and oil yield of rapeseed were increased significantly and the oil content percent was decreased to some extent, and the increase of N application rates led to an enhancementtrend in erucic acid content, and resulted in a slight decrease in glucosinolate content. But no significant differences in the contents of oil, erucic acid and glucosinolate were observed between the different application modes of N.Yield and N fertilizer contribution rate(NCR) increased with the increase of N application rates(90-270 kg/hm2), but apparently decreased when the N application rate reached to 360 kg/hm2. Agronomic efficiency of applied N(AEN),partial factor productivity of applied N(PFPN) and apparent recovery efficiency of applied N(REN) obviously decreased with the increase of N application rates. No significant differences were observed in yield,N fertilizer contribution rate(NCR) and apparent recovery efficiency of applied N(REN) between the rates of N 180 kg /hm2 and N 270 kg /hm2. Agronomic efficiency of applied N(AEN),partial factor productivity of applied N(PFPN) and apparent recovery efficiency of applied N(REN) significantly decreased when the N application rate was more than 270 kg/hm2 compared with the 90 kg/hm2 rate. The split nitrogen fertilizer application modes enhanced seed yield or oil yield and the twice of application mode as a basal fertilizer with a top dressing have a higher value under the same N application rate(225 kg/hm2). The values of nitrogen fertilizer use efficiency(AEN,PFPN,NCR,REN) in the treatment with the twice of application mode as a basal fertilizer and a top dressing were relatively higher than those in other N application modes. 【Conclusion】 The rate of N 180 kg/hm2 and the twice of application mode as a basal with a top dressing are suitable nitrogen fertilization mode under high planting density(36×104 plant/hm2)and direct-sown rapeseed in moderate soil fertility.