Abstract: Field trials were conducted to study the effects of different application time and basal / topdressing ratio of nitrogen fertilizer on the spatiotemporal variation of soil NO₃^--N and NH₄⁺-N contents and the grain yield and its quality of wheat. The results showed that soil NO₃^--N and NH₄⁺-N contents decreased with increasing soil depth. Both the application time and the basal / topdressing ratio of nitrogen fertilizer had significant effects on the NO₃^--N and NH₄⁺-N contents in 0-20 cm soil layer. Compared with basal application, later fertilization and higher topdressing ratio could promote the nitrogen uptake by wheat plant and increase the plant nitrogen recovery significantly, decrease the soil nitrogen recovery during wheat growth, and improve the grain quality significantly, while had less effects on the grain yield. Topdressing too much nitrogen fertilizer at booting stage could result in a significant decrease of grain yield. Under the condition of this experiment, the optimal nitrogen fertilization mode for good wheat grain yield and its quality and good ecological benefits was 5 ∶3∶2 of basal application ∶topdressing at jointing stage ∶topdressing at booting stage.

Key words: vegetation index, monitoring and diagnosis, spectrometer, plant nitrogen accumulation, double-cropping rice, model