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

Abstract In the present study, the effect and utilization of deep placement 15N-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. 15N-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). 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 15N 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 N2 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.