The mechanistic role of wildfire ash in regulating post-fire nitrogen transformation: a pathway as critically important as the thermal effects of fire

• Ash affects soil nitrogen transformation after fires through a hierarchical process. • Wildfire ash can synergistically alter soil properties and nitrogen transformation. • Wildfire ash does not affect the long-term soil nitrification process. • Regional fuel traits shape fire-linked soil nitrogen transformation. Wildfire ash plays a crucial role in mediating post-fire soil nitrogen (N) dynamics; however, the complex mechanisms that connect fuel characteristics, fire behaviors, ash properties, and soil N transformations are not yet fully understood. Wildfire ash samples were obtained by conducting 27 controlled combustion experiments (triple repetition) with different fuel loads (8, 12, and 16 t/ha) and moisture contents (5, 10, 15 %). Subsequently, a 49-day soil incubation experiment with the addition of wildfire ash was carried out. Through this controlled experimental system, we excluded the thermal effects of fire and post-fire erosion to determine the hierarchical impact mechanism of ash on soil N transformations and soil properties.The incorporation of wildfire ash resulted in distinct phases of mineral N dynamics, characterized by a rapid accumulation of NO 3 − -N within and a concurrent depletion of NH 4 + -N (7–21 days), contrasting with the delayed nitrification observed in the control samples. These changes were accompanied by transient biogeochemical alterations, including increases in soil pH, electrical conductivity (EC), and soil organic matter (SOM). This suggests that wildfire ash exerts a transient yet significant influence on soil N dynamics and properties, affecting mineral N transformations in a stage-specific manner without disrupting the natural nitrification process of the soil. The redundancy analysis highlighted the intertwined effects of physicochemical and biological regulation among fire, ash, and soil factors. Structural equation modeling revealed hierarchical controls, where fuel characteristics indirectly regulated net N mineralization, nitrification and ammonification via fire behavior and ash properties. This mechanistic framework positions wildfire ash as a biogeochemical engine, reshaping early-phase N transformation through alkali-driven substrate release and nitrifier activation. Our findings advance predictive models of post-fire N fluxes to balance ecosystem recovery with nutrient conservation.