Evaluation of N2fixation and N absorption activity by relative ureide method in field-grown soybean plants with deep placement of coated urea

In order to evaluate the beneficial effect of deep placement of coated urea fertilizer on soybean plant (Glycine max (L.) Merr. cv. Enrei) cultivation, a field experiment was conducted with three fertilizer N treatments: control, conventional basal dressing of 16 kg N ha-1 as ammonium sulfate; basal deep placement of 100 kg N ha-1 as 100 d type coated urea; and top dressing of 100 kg N ha-1 as 70 d type coated urea just before the flowering stage. The seasonal variation in the N2 fixation activity by the nodules and N absorption rates by roots were estimated by the determination of the relative ureide-N abundance in the root bleeding xylem sap by calculating the increase in total N accumulation. The N recovery rate from fertilizers was measured by the 15N tracer method. The N recovery rate in the plants with deep placement of coated urea was about 62%, and much higher than that of the plants with top dressing of coated urea (33%) at the R7 stage. Top dressing treatment depressed the N2 fixation activity during the maturing stage and the N absorption rate was higher than that of the control plants. On the other hand, deep placement of coated urea did not depress the N2 fixation activity throughout the growth stages until maturity, though N from fertilizer was actively absorbed. Consequently, the plants with deep placement of coated urea accumulated more N both from N2 fixation and N absorption than the plants with control and top dressing treatments. Seed yield was higher in the deep placement (592 g m-2) compared with the control (480 g m-2) and the top dressing (531 g m-2) treatments. The basal deep placement of coated urea for soybean cultivation appears to be beneficial by supplementing soybean N without concomitant depression of N2 fixation, and by increasing the seed yield. High recovery rate of fertilizer N is also desirable from the economical and ecological view points.