AimsThe degradation and transformation of soil organic nitrogen (SON) in semi-arid steppe are regulated by a series of enzymes involved in nitrogen(N) hydrolysis, the influence of N and carbon (C) additions on the soil N reserves, activities of N-hydrolyzing enzymes, and their relationships remain unclear.MethodsIn the Inner Mongolia prairie of China, a field experiment was conducted to study the effects of N (0, 25, 50, 100, 200 kg N ha-1 yr-1) and C (0, 250, 500 kg C ha-1 yr-1) additions on SON fractions and their relationships with N-hydrolyzing enzymes.ResultsOur results indicated that N addition significantly increased active-SON and N-acetyl-beta-D-glucosaminidase (NAG) activities and decreased urease activities. C addition significantly increased microbial biomass carbon (MBC), NAG, and urease activities, and decreased protease activity and hydrolyzable unknown-N. N and C additions interacted affected the microbial biomass nitrogen (MBN), MBC: MBN, protease, and amidase activities. Structural equation modeling suggested that N addition had a direct positive effect on hydrolyzable NH4+-N and amino acid-N. Furthermore, N addition indirectly affected amino sugar-N through MBN and the activities of NAG and protease. C addition directly affected urease activity.ConclusionOur findings suggest that active-SON responded significantly to N addition, whereas stable-SON did not. Moreover, N-hydrolysis enzymes, especially NAG and proteases, play a fundamental role in the N turnover under N and C additions in semi-arid steppe soils. As such, our work provides useful information for the development of sustainable steppe farming practices.