Differences of nitrogen absorption and utilization efficiency in direct seeding rice with different sowing date

Field experiments were conducted to compare the differences and relationships among yield,nitrogen(N) accumulation,N uptake rate and N use efficiency,with four representative rice cultivars of three types of early-maturing late japonica rice,late-maturing medium japonica rice and medium-maturing medium japonica rice(including japonica conventional rice and japonica hybrid rice) as experimental materials directly seeded at different sowing date.The results showed that: along with the delayed sowing date,(1) the yields decreased significantly or very significantly,which was caused by the decline of spikelets per panicle and filled grain percentage,while panicles and 1000-grain weight had a little change;(2) N content in plant increased in jointing stage increased but decreased in heading and maturity stage;(3) N uptake in plant of jointing,heading and maturity showed a significant downward trend;(4) N accumulation before jointing decreased,but its ratio to total rised,N accumulation and its ratio to total both from jointing to heading and from heading to maturity showed a gradual downward trend.(5) N uptake rate from sowing to jointing rised,but both from jointing to heading and from heading to maturity showed a significant or very significant downward trend.(6) N recovery N agronomic efficiency,N physiological efficiency and N partial productivity showed a downward trend of different degrees.(7) Correlation analysis indicated that there existed significant or very significant positive correlations between####