| Summary: | Sugarcane is the most promising among renewable biofuel-producing crops. An adequate availability of water and nutrients, especially nitrogen (N), is of utmost importance. From the hypothesis that the source and the rate of N fertilization influence the N use efficiency of an irrigated soil–sugarcane system, this study aimed to evaluate the stalk and crop residues (pointer and leaves) yield, as well as the N utilization, from different rates of ammonium nitrate and urea by irrigated sugarcane in the cane plant cycle, using an <sup>15</sup>N isotopic dilution technique. The soil used for the experiment was an Oxisol (Rhodic Haplustox) Cerrado (savannah) phase. The experiment was carried out at a farm of the Raízen Mill, which was located in the municipality of Jataí, Goias State, Brazil. The sugarcane variety IACSP95–5000 was used in a randomized block design, with three replicates, four N rates (30, 60, 120, and 180 kg ha<sup>−1</sup>), and two fertilizer sources labeled with <sup>15</sup>N isotopes (urea and ammonium nitrate). The average value for N utilization was 60.17% for the ammonium nitrate source and 44.95% for urea. The responses in the percentage and quantity of N in the sugarcane from the fertilizer increase with the increasing N rate. The soil was the main N source for sugarcane. The higher stalk and pointer yield was recorded at the average rate of 143.61 kg N ha<sup>−1</sup>. Further studies are needed to evaluate the residual effect of N in the next cycles.
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