Field experiments were conducted to evaluate the use of a novel ¹⁵N isotope technique for comparing the dynamics of N derived from sewage sludge applied to sorghum to the dynamics of N derived from the commercial fertilizer, urea. The treatments included a control, sludge applied at three rates (3, 6 and 9 t/ha, or 113, 226 and 338 kg N/ha) and N-urea applied at three rates (150, 250 and 350 kg N/ha). Recovery of ¹⁵N-labelled sludge was similar for the different nitrogen rates applied, with a mean value of 27%. However, the recovery of ¹⁵N-urea decreased as the rate of N application increased (from 38% to 27%). Approximately 22% and 19% of the ¹⁵N from sludge and urea, respectively, remained in the 0-60 cm layer of soil, most of which was present in the 0-20 cm layer. Furthermore, losses of ¹⁵N-labelled fertilizer were not affected by the N fertilization source, and the greatest losses, which were measured in response to the highest N application rate, were 59%.