The objective of these studies is to increase the productivity of soybean [ Glycine max (L.) Merr. cv. Enrei] with good quality by the application of controlled release nitrogen fertilizers. Soybean was grown in an upland field converted from a drained paddy field in Niigata. The fertilizer treatments were as follows: (a) control, conventional basal dressing of ammonium sulfate (16 kg N/ha); (b) deep placement, deep placement of coated urea, 100 day type (100 kg N/ha) with basal dressing of ammonium sulfate (16 kg N/ha); (c) top dressing, top dressing of coated urea, 70 day type (100 kg N/ha) just before the flowering stage with basal dressing of ammonium sulfate (16 kg N/ha).We employed the following methods and materials: "lSN-labeled fertilizers," to estimate the amount of N derived from fertilizers; "N-balance method" using the nodulating and non-nodulating isogenic line cv. T202 and T201, to estimate the amount of fixed N~ by root nodules; "relative ureide method" to determine the relative content of ureide-N in the root bleeding sap, in order to estimate the activity of the root nodules.The accumulation of dry matter and total N was the highest in the plants with the deep placement treatment, in which the promotion of leaf growth and retardation of senescence were observed during the maturation stage. The seed yield in this treatment which was 424 g/m 2, surpassed those in the control (373 g/m 2) and the top dressing (359 g/m s) treatments. The absorption efficiency of fertilizer N in the deep placement treatment was 49~o, which was much higher than that in the top dressing treatment (27~o). However, the contribution of fertilizer N to the total N absorption was found to be low at 16~o in the deep placement, and 11~o in the top dressing treatment at the R7 stage. In the deep placement treatment, a marked increase in N accumulation in shoots was observed especially from the R3 till R7 stages. The N-balance of the nodulating and non-nodulating cv. T202 and T201 at the R7 stage suggested that more than 80% of the accumulated N was derived from the fixed N2 by root nodules. On the other hand, in the top dressing treatment, the amount of N accumulated during the R3 to R7 stages was lower than that in the other treatments, possibly due to the depression of N2 fixation by top dressing.Abbreviations: DW: dry weight; cu-100: coated urea, I00 day type; cu-70: coated urea, 70 day type; Cont.: control; Deep pl.: deep placement of cu-100; Top dr.: top dressing of cu-70; LAI: leaf area index; DAS: days after sowing. Y. TAKAHASHI et al.The results suggested that the deep placement of coated urea as basal application improved soybean growth, and did not depress markedly the nitrogen fixation activity during the maturation stage, hence the increase of seed yield. Top dressing of coated urea inhibited the nodule activity after the R3 stage, and consequently the seed yield did not increase.

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Soybean is an important crop for human food and feed for livestock. World soybean production is increasing especially in North and South America. Soybean seeds contain a high percentage of protein about 35-40%, and they require a large amount of nitrogen compared with other crops. Soybean plants make root nodules with rhizobia, and rhizobia can fix atmospheric N 2 and give the fixed N to the host soybean plants. Also, soybean can absorb nitrogen usually nitrate from soil or fertilizers. The amount of total assimilated nitrogen in shoot is proportional to the soybean seed yield either from nitrogen fixation or from nitrogen absorption, and the nitrogen availability is very important for soybean cultivation. Maintenance of a high and long-term nitrogen fixation activity is very important for a high production of soybean. However, application of chemical nitrogen fertilizers usually depresses nodule formation and nitrogen fixation. Nitrate in direct contact with a nodulated part of roots causes severe inhibition of nodule growth and nitrogen fixation, although a distant part of nodules from nitrate application gives no or little effect. Deep placement of slow-release nitrogen fertilizers, coated urea, or lime nitrogen promoted the growth and seed yield and quality of soybean without depressing nitrogen fixation.

In the present study, the effect and utilization of deep placement 15 N-labeled coated urea (CU) and lime nitrogen (LN) were investigated at various growth stages of soybean (Glycine max [L.]). Soybean (cv. Enrei) plants were planted in a rotated paddy field in Niigata, Japan. ) was supplied separately at a depth of 20 cm just below the seeding line. Deep placement of LN and CU significantly enhanced the dry weight and N content of the plants compared with the control treatment (Cont) without deep placement. As a result, the seed yield per plant in CU (67.2 g) and LN (70.6 g) was much higher than the Cont (37.1 g). 15N analysis of plants showed that the pattern of labeled N absorption tended to be lower with LN than CU at the R3 and R5 stages, but the recovery rate at R7 was higher in LN (70%) than CU (61%). Combining the 15 N analysis with a relative ureide analysis, the N derived from nitrogen fixation (Ndfa), from fertilizer (Ndff ) and from soil (Ndfs) were evaluated. At the R7 stage, the amount of Ndfa was higher in CU (32.1 g m -2 ) and LN (31.1 g m -2 ) than in Cont (21.4 g m -2 ). This positive response for N 2 fixation by the deep placement of CU and LN may result from the continuous supply of N from the lower parts of the roots, which promotes shoot growth and extends the photosynthetic activity of the green leaves, resulting in the promotion of nitrogen fixation and seed yield.

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