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 20th 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.
