Background and aims <p>The establishment of constructed grasslands is a widely used method for restoring severely degraded alpine grasslands in the Three-River Headwaters Region (TRHR). The response of soil net nitrogen mineralization (SNNM) to grassland construction for ecological restoration and how this response changes with recovery ages remain unclear.</p> Methods <p>One-way ANOVA, the Tukey’s HSD test, ordinary linear regression and PLS regression were employed to analyze the differences in SNNM rate between constructed and black-soil-type grasslands, their relationships with soil properties, and their key influencing factors.</p> Results <p>The SNNM rate in constructed grasslands exhibited a hyperbolic trend of initial surge– decrease–low-level stabilization, whereas those in black-soil-type grasslands indicated no clear temporal trend. Soil net nitrification dominated the SNNM processes in both grassland types, accounting for 94.3% and 93.1% of the SNNM rates in constructed and black-soil-type grasslands, respectively. Changes in soil chemical properties, particularly carbon, nitrogen, and pH, significantly influenced the differences in SNNM rate between constructed and black-soil-type grasslands (<i>P</i> &lt; 0.05). Soil total nitrogen, soil available nitrogen, and microbial biomass nitrogen had the greatest effects (28.6%), followed by soil microbial biomass carbon and organic matter content (27.1%). Changes in soil physical properties, including bulk density, water content, and temperature, did not significantly influence SNNM rate changes.</p> Conclusion <p>Research on constructed grasslands in the TRHR demonstrated that their establishment for alpine ecological restoration induces an initial increase in the SNNM rate, followed by a decrease, even under organic fertilization application. This suggests potential risk of fertility decline and secondary degradation, as well as the riverine nitrogen pollution associated with constructed grasslands expansion.</p>