Soil extractable organic nitrogen (SON) is an important nutrient pool involved in N transformations, and the content and conversion of SON are affected by fertilization practices. However, many gaps remain in our understanding of SON, especially in agricultural soil. The effects of long-term (1990-2009) fertilization on SON at elongation, flowering, and harvest stages in wheat were evaluated in a loess soil (Eum-Orthic Anthrosol) in northwestern China. The treatments included no fertilization (No-F), application of inorganic NPK fertilizer (NPK), and combined application of inorganic NPK and manure (MNPK). Using ¹⁵N tracer techniques, ¹⁵N-labeled urea (165 kg N/hm²) was applied to microplots within each treatment to investigate the effect of short-term addition of N on content of SON during the wheat-growing season in wheat monoculture cropping systems. The SON content was 7.5-29.3 kg/hm² and accounted for 40%, 56%, and 56% of total extractable N in No-F, NPK, and MNPK, respectively. Compared with No-F, application of inorganic NPK fertilizer increased SON content significantly (55% on average) in the 0-15 cm soil layer. Soil extractable organic N content in the MNPK treatment was significantly higher (by 32%-35%) than that in the NPK treatment in the 0-15 cm layer. Long-term fertilization had no effect on SON content below 30 cm. SON was highest at flowering and was significantly higher during flowering than at the elongation stage in NPK and MNPK (by 48% and 82%, respectively). In relation to No-F, fertilization treatments increased the SON significantly in the 0-100 cm soil profile, SON was 43.1, 51.6, 55.2 kg/hm² in No-F, NPK, and MNPK, respectively. Addition of N had no significant effect on SON content in the 0-15 cm soil layer during the same growing season; however, 0.4%-2.8% of SON was derived from the ¹⁵N-labeled fertilizer applied before seeding, representing 0.03%-0.24% of the fertilizer, and short-term addition of N increased SON in the 0-100 cm soil profile by 35%, 30%, and 14% in No-F, NPK, and MNPK, respectively. We conclude that the conversion of inorganic N to extractable organic N was slow. However, long-term fertilization increased SON content in the topsoil, and SON is a significant nitrogen pool in agriculture soils.