Recovery of N-15-labeled fertilizer by spring bread wheat at different N rates and application times

Optimal N fertilization can improve the yield and quality of spring bread w heat in eastern Canada. This study aimed to determine the economical N rate for the production of spring bread wheat (Triticum aestivum L. 'AC Pollet' ) and to compare the effect of application times on the efficiency of ferti lizer N use. The experiment was conducted during 2 yr on two sites of Saint e-Rosalie clay loam. The experimental treatments were arranged in a split-p lot design with N rates (0, 30, 60, 90, 120, 180 kg ha(-1)) as the main plo ts and the application times of fertilizer N ((NH4NO3)-N-15-N-15 applied at seeding and booting stages) as the subplots. Grain yield, grain protein co ncentration and straw N content of wheat were increased significantly with N application rates. The economic N rates were 90 and 120 kg ha(-1) for 199 3 and 1994, respectively. The recovery of N-15-labeled fertilizer (%FNR) in grain and straw was higher when applied at booting stage than at seeding i n both years. In 1993, FNR varied from 37.8 to 45.7% for seeding and from 6 2.1 to 68.4% for booting stage treatments. The respective values were 23.1 to 30.4% and 41.3 to 50.7% in 1994. At each N rate, the proportion of N der ived from fertilizer (Ndff) was higher in grain than that in straw when N-1 5 fertilizer was applied at booting stage. The combined recovery of N-15 fe rtilizer (% total FNR)) applied at seeding and booting, as determined by th e isotopic and the difference method, was in the same range, with a mean of 49.8% and 36.2% for 1993 and 1994, respectively. Soil N supplies for wheat during the growing season were 54 and 61 kg N ha(-1) in 1993 and 1994, res pectively. No priming effect of added fertilizer N on the mineralization of soil N was observed.