Abstract: Nitrogen fertilizer management plays an important role in increasing crop yield and reducing fertilizer inputs. A field experiment was conducted to evaluate the effects of nitrogen application rates and time on dry matter accumulation, transport, distribution and grain yield of flax in the dryland of northwest China. There were three nitrogen rates (N1: 60 kg·hm^-2; N2: 120 kg·hm^-2; N3: 180 kg·hm^-2) and three nitrogen application periods (T1: full basal application; T2: 2/3 basal application + 1/3 topdressing at the beginning of budding; T3: 1/3 basal application + 1/3 topdressing at the beginning of branching + 1/3 topdressing at the beginning of budding). The results showed that dry matter accumulation, preanthesis dry matter transport, and its contribution rate to the grain of flax at the nitrogen rate of 120 kg·hm^-2 (N2) were significantly higher than those of other two nitrogen rates by 5.47%, 34.87% and 6.63% in 2020, and 4.00%, 17.57% and 4.54% in 2021, respectively. Grain yield of treatment N2 was higher than that of the other treatments by 4.29% in 2020 and 5.11% in 2021, respectively. Under different fertilization periods, the treatment (T2) of 2/3 basal application + 1/3 topdressing at the initial budding stage significantly promoted flax growth and development. The interactive effects between nitrogen application rate and application period on dry matter accumulation, dry matter distribution at maturity, pre-anthesis dry matter transport, and post-anthesis dry matter accumulation were significant. Treatment N2T2 had significantly higher dry matter accumulation and grain dry matter distribution than the other combination treatments. Treatment N2T2 significantly increased pre-anthesis dry matter transport, post-anthesis dry matter accumulation and grain yield by 49.43%, 24.52%, and 7.29% in 2020 and 37.30%, 9.90%, and 9.11% in 2021 respectively, compared to the traditional nitrogen application (N3T1). There was a significant and positive correlation between dry matter accumulation, grain yield, and yield components of flax. The increases of effective fruit number and grain number per fruit were the main reasons for the increase of flax grain yield. Our results indicated that the treatment of 120 kg·hm^-2 nitrogen application rate with 2/3 basal application + 1/3 topdressing at the beginning of budding promoted dry matter accumulation of flax, increased the proportion of dry matter in grains and dry matter allocation of grains at the anthesis stage, which in turn increased the number of effective fruits and the number of grains per fruit, thus improving grain yield of flax. This nitrogen fertilizer management method was suitable to achieve high flax yield.

Key words: nitrogen fertilizer management, flax, dry matter accumulation and distribution, grain yield.