What is the role of fire in the long-term nitrogen (N) budgets in Sierran Forests? The answer to this question is not straightforward. The immediate effects of a fire are to cause losses of N by volatilization and mobilization of organic N to ammonium form. Over the longer term, however, fire may indirectly cause long-term increases in soil N by stimulating post-fire N fixation.

Results/Conclusions

A review of fire and N budget studies in forests of the eastern Sierra Nevada indicates that N losses by fire exceed N fluxes by atmospheric deposition and leaching by at least two orders of magnitude. Wildfire causes much greater N losses than prescribed fire during any single episode, but the cumulative effects of repeated prescribed fire can theoretically cause greater long term N losses than a single wildfire. Post-wildfire N fixation over two decades in one study site more than replenished lost N, leading to the hypothesis that wildfire could actually indirectly cause long-term increases in ecosystem N pools if post-fire N fixation occurs. Prescribed fire at short intervals precludes the establishment of N fixers, however, and could lead to long-term declines in ecosystem N pools (as may have been the case prior to 20^th century fire suppression). Finally, our research suggests that buildups of forest floor material due to fire suppression has caused increases in N- and P-enriched surface water flow through O horizons, potentially having deleterious effects on water quality.