Nitrogen (N) is a main limiting factor for plant growth and development in many regions. Soil organic N, accounting for up to 95% of the soil N pool, cannot be used by plants directly, but can be absorbed and utilized by plants after being mineralized into nitrate N (NO₃^--N) and ammonium N (NH₄⁺-N). Therefore, soil net N nitrification (N_nit) and net N mineralization (N_min) rates have been considered two important indicators for evaluating the stability and function of an ecosystem. Caragana microphylla, a leguminous and sand-binding shrub, is a dominant species widely distributed in the Loess Plateau of northwestern Shanxi. Previous studies on soil N dynamics have mainly focused on forest, grassland, and farmland ecosystems to study the effect of environmental factors such as temperature and humidity on soil N mineralization and nitrification. However, few researchers have focused on soil N dynamic changes under shrub lands in arid and semiarid regions. The objective of this study was to explore the content of NO₃^--N and NH₄⁺-N and the rate of soil net N nitrification and net N mineralization under Caragana microphylla shrubs as a function of plant age. We used a closed-top PVC tube in situ incubation method to detect the soil net N mineralization and net N nitrification in Caragana microphylla shrubs of different ages (5, 10, 20, 30, and 40 a) in the northwest Shanxi Loess Plateau. The results indicated that NO₃^--N accounted for a larger proportion of mineral N in Caragana microphylla shrub soil. The NO₃^--N concentration was 1.5-15.4 times higher than NH₄⁺-N in the same month of different years. Soil net N nitrification and net N mineralization rates increased as the Caragana microphylla shrubs' ages increased, with the greatest average rates from May to October (40.2 and 44.1 mg m^-2 d^-1, respectively) being observed under 30-year-old shrubs. The net N nitrification and mineralization rate increased faster in July and August relative other months during the growing season. The net N nitrification and mineralization rates were 86.9 and 93.1 mg m^-2 d^-1, respectively, at 30-year-old shrubs, which had the highest rates of all age classes (P < 0.05). The total net N nitrification and mineralization also increased with increased age of Caragana microphylla. The total net N nitrification and mineralization from May to October increased by 3.7- and 3.1-fold at 30-year-old shrubs, respectively, relative to 5-year-old shrubs. During the Caragana microphylla growing season from May to October, 2.3% of soil total N was mineralized into inorganic N, 87% of which was eventually transformed into NO₃^--N. The results of this study provide great insight for guiding shrub utility and land management in the establishment and development of shrub ecosystems in the Loess plateau.