Effect of methods of nitrogen application on nitrogen recovery from ¹⁵N-labeled urea applied to paddy rice (Oryza sativa L.)

Nitrogen efficiency of two methods of application of urea at topdressing of paddy rice (Oryza sativa L. cv. Koshihikari) before panicle initiation stage (BPIS) as well as recovery of basally applied nitrogen and topdressed nitrogen BPIS were studied in a pot experiment. Three N rates (200 (N₂₀₀), 400 (N₄₀₀) and 600 (N₆₀₀) mg kg^-1 sou) and equal P₂O₅ and K₂O rates (150 mg kg^-1 soil) were applied. Topdressing BPIS was made by broadcasting ¹⁵N-labeled urea and by injection of ¹⁵N-labeled urea solutions in depth of 8 cm, while the basal fertilization was made with unlabeled urea. For other three treatments with the same N rates, ¹⁵N-labeled urea only for basal application was used. Concentrations of the ammonium in leaching water varied from 45 mg NH₄-N L ^-1 (N₂₀₀) to 192 mg NH₄-N L^-1 (N₆₀₀) one week after transplanting date (TD) of rice and dropped to zero for about a month. The amount of urea determined a week after TD was 3, 9 and 13mg urea-N per pot at the three N rates, respectively. The concentration of potassium in the leaching water depended on N rates (higher N rate led to higher leaching of K) and plant growth. No positive effect was found of the injection method of urea application into the soil on the grain yield and the nitrogen accumulation in the plants in comparison with the broadcasting method of urea application BPIS. The differences in ¹⁵N recovery among studied N rates were less with injected urea BPIS where ¹⁵N recovery was from 68% at the lowest N rate to 75% at the highest rate, while that was from 57 to 79%, respectively with broadcast urea BPIS. The recovery of basally applied nitrogen was less than topdressed nitrogen and ranged from 39 to 44%. There was no significant difference between methods of N application BPIS on the apparent nitrogen recovery, which varied between 58 and 68%.