Economical Nitrogen Application on Late Sowing Wheat With Rice Straw Returning Under Rice-Wheat Rotation

The straw of preceding rice was returned to field totally,and winter wheat was sowed with five N rate treatments（ 0,90,180,270 and 360 kg·hm-² applied to investigate the mineral N accumulation,straw N release,N balance and grain yield of winter wheat. The results show that mineral N accumulation amount in 0- 30 cm soil layer at overwintering stage increased with basal N application rates,as increased with top dressing N application rates in 0- 15 cm soil layer at anthesis stage. Mineral N residual was significantly higher than before planting when nitrogen rate was applied higher than180 kg·hm^-2 .The analysis on N balance further indicat that the soil net mineralization N was 48 kg·hm^-2 the amount of straw N release,plant N uptake,mineral N residual and apparent N losses increased as nitrogen application rate increased. The N loss ratio of N90,N180,N270 and N360 treatment was 27. 9%,37. 6%,43. 2% and 47. 6%,respectively. The loss of N mainly occurred before overwintering stage in N90 treatment,while in other treatments it was found after anthesis. There was no significant increase in yield when nitrogen rate exceed 180 kg·hm^-2 .Considering both the grain yield and ecological effect,we suggest that the proper amount of nitrogen application for late sowing wheat with total preceding rice straw returning is 180 kg·hm^-2 in those growing areas similar to this experimental condition.