Background and aims <p>Afforestation is widely used to restore degraded lands and enhance soil fertility in high-altitude regions. However, it remains unclear whether high-altitude ecosystems share consistent seasonal patterns in soil ammonium-nitrogen (NH₄⁺-N) and nitrate–N (NO₃⁻-N), and how stand age and season jointly regulate inorganic N retention through microbial and enzymatic&#xa0;pathways in Tibetan Plateau plantations.</p> Methods <p>This study integrated a global meta-analysis with field investigations in <i>Salix matsudana</i> plantations of three developmental stages (young, middle and old) across growing and non-growing seasons on the Tibetan Plateau to examine soil inorganic N dynamics and the associated microbial mechanisms.</p> Results <p>Meta-analysis revealed higher NH₄⁺-N in the growing season, whereas NO₃⁻-N exhibited no consistent seasonal trend. Field observations showed that middle-aged stands had the highest soil organic matter, microbial biomass N and NH₄⁺-N, but lower NO₃⁻-N across seasons. Functional genes related to dissimilatory NO₃⁻ reduction to NH₄⁺ (<i>nirB</i>) and denitrification (<i>narG</i>) were most abundant in middle-aged forests, supporting NH₄⁺-N accumulation. Seasonal shifts modulated microbial regulation strategies: the growing season favored a microbe-environment synergy, with Proteobacteria promoting N mineralization and community stabilization; in contrast, the non-growing season was characterized by enzyme-environment coupling, wherein persistent enzyme activity and functional redundancy maintained N transformation.</p> Conclusions <p>These findings reveal a seasonal reorganization of N-regulating pathways, with control shifting from microbe-dominated processes in the growing season to enzyme-dominated regulation in the non-growing season. This mechanism is most pronounced in middle-aged plantations, identifying a critical management window for enhancing soil N retention in high-altitude afforestation systems.</p>