COS 166-1 - Wildfires increase soil emissions of NO_x but not N₂O along an ash depth gradient in California chaparral

Background/Question/Methods

Wildfires cause large-scale ecosystem Nitrogen (N) loss by volatilizing N bound in plant biomass and leaving behind a layer of ammonium (NH₄⁺) rich ash. This remaining N is susceptible not only to losses via leaching and runoff, but N loss may continue as post-fire soil physicochemical conditions promote microbially-driven emissions of nitric oxide (NO) and nitrous oxide (N₂O)—trace gases that alter air quality and climate. Temporarily elevated soil N availability and pH may stimulate nitrification and denitrification, accelerating NO and N₂O release from soils. As the amount of ash likely governs both N availability and pH, we hypothesize that ash depth is a strong predictor of soil NO and N₂O emissions post-fire. To test this, we measured ash depth at 36 plots two weeks after the Holy Fire burned in the Cleveland National Forest, CA, and sampled soils seasonally over three years post-fire. We monitored changes in soil N availability, microbial biomass, and pH along with measuring NO and N₂O emissions in soil microcosms. By understanding the correlations between gas fluxes and ash depth we hope to develop tools for quick assessment of potential post-fire N loss and trace gas emissions relevant to land managers.

Results/Conclusions

Soil NH₄⁺ increased by a factor of 4 and pH by 0.6 in burned plots over the study period but approached unburned levels in year three. Deeper ash layers corresponded significantly with higher pH (R²=0.09, p=0.03), but only weakly with NH₄⁺(R²=0.02, p=0.26). Despite high variability, burning increased soil emissions of NO (p=0.04), with cumulative fluxes over a 48hr incubation period reaching 154.9 ± 110.8 ng N-NO g^-1 in burned plots and 59.68 ± 49.29 ng N-NO g^-1 in unburned plots. There was no significant effect of burning on N₂O emissions (p=0.38) which remained low across all plots. These results indicate that fires have the potential to accelerate NO emissions on long timescales due to high soil N availability and pH, conditions which are known to promote the activity of ammonia oxidizing bacteria and are associated with high NO production. Preliminary regression analyses indicate that NO fluxes are better predicted by pH (R²=0.25, p=0.028) and NH₄⁺ (R²=0.27, p=0.023) individually than ash depth (R²=0.042, p=0.39). However, because it is tightly linked with soil pH, ash depth may still prove a helpful predictive tool for soil NO emissions as this data set is expanded.