Irrigation is one of the most important measures to regulate plant metabolism in winter wheat(Triticum aestivuml). However, unsuitable irrigation managements would cause the decline of nitrogen use efficiency and grain yield, and increase NO₃-N content in deep soil layers. The field experiment was carried out to investigate the effects of delayed irrigation at jointing stage on nitrogen accumulation and its allocation, and NO₃-N migration in wheat with the high-yielding cultivar, Jimai 22. Soil water content was controlled by a new strategy that supplemental irrigation based on measuring soil moisture. In 2007-2008 growing season, the first irrigation was given at jointing stage or 10 d after jointing stage, and the soil relative water content in 0-140 cm soil layer arrived to 65%, 75%, and 80% after irrigations, respectively. The treatments irrigated at jointing stage are represented as W1, W2, W3, and treatments irrigated 10 d after jointing stage are represented as DW1, DW2, DW3. The second irrigation was given at anthesis stage, and the soil relative water content of all irrigation treatments reached to 70% after irrigation.
In the treatments irrigated at jointing stage, the nitrogen translocation amount from vegetative organs to grains, nitrogen allocation amount into kernel at maturity stage, grain yield, nitrogen uptake efficiency and partial productivity of nitrogen applied were significantly higher in treatment W2 than in treatment W1, and there was no significant difference between those of treatment W2 and treatment W3. Compared with treatment W3, however, treatment W2 increased the NO₃-N content in 0-60 cm soil layer, plant nitrogen accumulation amount post anthesis, grain protein content, and water use efficiency (WUE), significantly. In the treatments irrigated 10 d after jointing stage, the nitrogen translocation amount from vegetative organs to grains, grain yield, and partial productivity of nitrogen applied were higher in treatment DW2 than in treatment DW1, and there was no significant difference between those of treatment DW2 and treatment DW3. While the NO₃-N content in 0-60 cm soil layer, plant nitrogen accumulation amount post anthesis, nitrogen allocation amount into kernel at maturity, grain protein content, nitrogen uptake efficiency and WUE were higher in treatment DW2 than in treatment DW3. Compared with treatment W2, treatment DW2 decreased the NO₃-N content in 100-140 cm soil layer at maturity stage, and increased the grain yield, grain protein content, nitrogen uptake efficiency, partial productivity of nitrogen applied and WUE, significantly. As a result, treatment DW2 may be considered to be the best irrigation regime under the current experiment condition. The change tendency of different treatments in 2008-2009 growing season was similar to that in 2007-2008 growing season.