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In order to identify effects of nitrogen (N) and phosphorus (P) on soil nitrogen morphological characteristics and grain yield of oil flax,a two-factor experiment was conducted in a randomized complete block design in typical semi-arid and hilly-gully area of Loess Plateau with 3 replicates in 2013 and 2014.Two levels of N application included 150 kg/hm2 (N2) and 75 kg/hm2 (N1).P application included 150 kg PaO5/hm2 (P2) and 75 kg P2O5/hm2 (P1).Temporal and spatial variation of soil nitrate nitrogen (NO3--N) and ammonium nitrogen (NH4+-N) contents in 0-60 cm soil layer,and relationship between soil NO3--N accumulation (SNA) and grain yield of oil flax were analyzed.Results showed that SNA increased with evaluated N application rate in different soil layers (0-20 cm,20-40 cm and 40-60 cm).With the increased P application,SNA increased at N1 level but decreased at N2 level.SNA under N2P1 treatment increased by 73.33％ in 2013 and 74.97％ in 2014 respectively,compared with control treatment (CK) at maturity stage.Grain yield of oil flax also increased by 44.27％ in 2013 and 56.55％ in 2014,compared with CK under the same treatment.Correlation analysis showed that SNA in different soil layers were respectively positively correlated with grain yield.In conclusion,this research suggested that the optimal fertilizer application rate was 150 kg N/hm2 and 75 kg P2O5/hm2 in the Northwest of China.