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Soybean (Glycine max [L] Merr.) plants can utilize N2 fixed in the root nodules. Also, they can use the N absorbed from roots, from soil mineralized N or from fertilizer N when applied either in the form of nitrate or ammonium (Ohyama et al. 1989). However, the nodulation and nitrogen fixation of soybean is depressed by the application of combined N such as chemical N fertilizers. Takahashi et al. (1991) initially reported the promoted effects of deep placement of slow release N fertilizer, 100-day type coated urea on soybean seed yield. Deep placement of slow release N fertilizers, such as coated urea and lime nitrogen promotes soybean growth and seed yield without depressing N2 fixation by root nodules (Tewari et al. 2002, 2003, 2004). In the present study, the effect of deep placement of various fertilizers was compared by the analysis of N derived from atmospheric N2 (Ndfa), N from soil (Ndfs) and N from fertilizer (Ndff) in the rotated paddy field of Niigata Agricultural Research Institute using N dilution method. |
