By applying labeled urea into a loamy meadow brown soil,a pot experiment with spring wheat as test crop was carried out. The results showed that in comparison with applying urea alone and urea+hydroquinone(HQ),an application of dicyandiamide(DCD),and especially of its combination with HQ, gave a much higher recovery of soil urea ¹⁵N, among which organic ¹⁵N occupied a great portion.These two treatments played an active role in the conservation of urea-deleased NH₄⁺-¹⁵N in soil and in the decrease of the accumulation of urea-derived (NO₃^-+NO₂^-)-¹⁵N. They could markedly promote the mineralization-immobilization turnover of fertilizer ¹⁵N following the hydrolysis of applied urea Synergistic effects of HQ and DCD on the contents of fertilizer-¹⁵N and NH₄⁺-¹⁵N in soil as well as the mineralization of soil organic-¹⁵N will appear after the application of labeled urea.