Analysis of the nitrogen nutrition of soybean plants with deep placement of coated urea and lime nitrogen

In the present study, the effect and utilization of deep placement N-15-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. N-15-labeled CU or LN (100 kg N ha(-1)) 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). N-15 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 N-15 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.