In China, greenhouse soils often receive large rates of different manures and have a high content of soil organic matter (SOM). Understanding changes in nitrogen (N) mineralization in soils of newly built greenhouses after their construction is important for managing N. Soil samples were obtained from solar greenhouses of different ages (0, 1, 2, and 3 years) located in the south edge of the Loess Plateau, China, at 0- to 20- and 20- to 40-cm depth. N mineralization in the soils was measured with the Stanford and Smith long-term aerobic incubation method over 30 weeks. SOM, total N, and the mineralized N in the 0- to 20-cm and 20- to 40-cm soil layers were significantly increased in the older greenhouses. The cumulative mineralized N in the 0- to 20-cm soil layer in different cultivation years was increased in each year since the greenhouses were established. For the greenhouses with the same age, the cumulative mineralized N in the 0- to 20-cm soil layer was greater than that in the 20- to 40-cm layer. The potentially mineralizable N (N₀) both in the 0- to 20-cm and the 20- to 40-cm soil layers increased with the greenhouses’ age. Regression analysis indicated that when SOM increased 1 g·kg⁻¹, N₀ in the 0- to 20-cm and 20- to 40-cm depth increased 22.6 and 8.4 mg·kg⁻¹, respectively. Therefore, as the N supply in soil increases with the age of the solar greenhouse, we suggest that the application rates of manure and synthetic fertilizer be reduced.