Long-term fire effects on soil and vegetation nitrogen cycling: Potential links to persistent stream nitrate export

Background Soil and stream nitrate (NO₃⁻) concentrations often increase after severe fire from elevated nitrogen (N) mineralization and reduced plant uptake. However, it is unclear how long these effects persist and contribute to stream N export. Aims We examined the contribution of soil N supply and vegetation N demand to 19-fold higher stream NO3- export that has persisted since the 2002 Hayman Fire in Colorado. Methods We compared soil N pools, inorganic N production, subsurface (0-100 cm) NO3- concentrations, and vegetation cover, productivity, and N demand 17 years post-fire. We sampled along burned and unburned hillslopes to evaluate whether near-stream vegetation and soils attenuated N loss during downslope transport. Key results Mineral soil, leachate, and groundwater NO3- concentrations were higher in burned than unburned hillslopes, despite similar mineralization rates. Burned uplands showed 62% lower productivity and 28% lower N demand relative to unburned forests. Riparian recovery exceeded uplands but remained incomplete relative to unburned conditions. Burned uplands acted as N sources, with slight reductions in soil NO3- in downslope riparian soils. Conclusions Sustained NO3- export was driven by reduced vegetation N demand and subsurface transport, not increased mineralization. Implications Revegetation of severely burned uplands and riparian zones may enhance long-term N retention.