Application of nitrogen (N) fertilizer is one of the most important measures that increase grain yield and improve grain quality of winter wheat (Triticum aestivum L.). However, it is common that excessive N fertilizer is applied on high fertility field, which not only causes the decline of N use efficiency and economic effects, but also results in larger amount of nitrate-N accumulated in soil, and so it is concerned to bring potential risk to environment. This research was conducted to determine the effects of N fertilizer rate on N fertilizer use, grain yield and quality of winter wheat, and changes in soil nitrate-N content during wheat growth period under high-yielding conditions. The results will help to choose optimum N rate, to obtain profitable yields, efficient N use, and reduce the possible impact on environment.
The experiments were carried out in two high-yielding fields, differed in soil fertility in Qianzhuliu, Longkou, Shandong, China, during 2002～2003. Seven N treatments (CKI, CK2, N1, N2, N3, N4, N5) were designed, with the N rate of 0, 0, 105, 150, 195, 240, 285 kg/hm², respectively. The N fertilizer was urea. Half of the N fertilizer was applied before sowing, and the other was topdressed at jointing stage. Apart from the treatment of CK1, all the other 6 treatments were supplied with 135 kg P₂O₅ /hm² and 105 kg K₂O/ hm² before sowing. Each treatment had 3 replicates, with a plot of 3m×8m. In the field plots of N1, N3, N4 treatments, microplots of 15N tracing experiments were set, with the area of 15cm×44.5cm, and isolated with a 30cm high iron frame. Each ¹⁵N microplot had 2 replicates, which was added with 10.13 atom% 15N-urea before sowing or topdressed. The rate and date of N application in the microplots were the same to these in the field plot. Wheat was sowed on October 8 in 2002, with plant density of 120/m².
The results showed that 73.32%～87.27% N accumulated in wheat plant at mature stage was derived from soil, 4.51%～9.40% from basal N fertilizer, and 8.22%～17.28% from topdressed N fertilizer. With an increase of N fertilizer rate, soil N accumulated in wheat plant decreased, and fertilizer N that accumulated in wheat plant and remained in soil increased, whereas the recovery rate of fertilizer N decreased significantly. The amount of N that accumulated in wheat or remained in soil derived from basal fertilizer was less than that derived from topdressed fertilizer. The amount of N loss of basal fertilizer was more than that of topdressed fertilizer. Compared with the lower fertility field, the amount of soil N absorbed by wheat increased, whereas that of fertilizer N decreased, and the amount of fertilizer N remained in soil or became loss increased in the higher fertility field. The differences in plant uptake, soil residual and loss of fertilizer N between the two fields came mainly from those of the basal N fertilizer. With N application of 105 kg/hm², no larger amount of nitrate-N was found accumulated in 0～100cm soil layers at mature stage. With N fertilizer rate increasing, the content of nitrate-N in 0～100 cm soil layers increased dramatically. With more than 195 kgN/hm² was supplied, soil nitrate-N moved down obviously, and the upward nitrate-N movement occurred earlier and deeper in the higher fertility field than that in the lower one. With N fertilizer rate increasing, N uptake efficiency and N utilization efficiency of wheat decreased. With appropriate rate of N fertilizer added, grain yield, protein content and the amount of N accumulated in wheat plant all increased. High grain yield (＞8800 kg/hm² ) and high protein content (＞14.20%) could be obtained in both the fields. However, with application of excessive N fertilizer, grain yield and protein content would increase little, and even decreased. The N fertilizer rates with the highest grain yield or the highest protein content in the higher fertility field were lower than that in the lower one. Based on analysis of grain yield, quality, N utilization and soil nitrate-N content, the N fertilizer rates recommended in the two fields were 105～150 kg/hm² and 150～195 kg/hm², respectively.