Fires can drive ecosystem nitrogen (N) loss and contribute to N limitation by volatilizing N bound in plant biomass. Fires may also contribute to ecosystem N loss by raising soil pH and leaving behind ash rich in NH4+ and organic N. While some of this N can be lost through hydrologic pathways, budget imbalances suggest soil emissions of nitric oxide (NO) and nitrous oxide (N2O) may become important after fire. Soil NO and N2O emissions are tightly correlated with the activity of nitrifying organisms such as ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA), raising the question of how the individual responses of these groups to increased pH and NH4+ following fire might impact NO and N2O emissions. Here we hypothesize that the elevated NH4+ and pH caused by wildfire ash will favor AOB over AOA, increasing emissions of NO and N2O in burned soils. To address this, we collected soil cores at multiple timepoints from a network of 36 plots in the Cleveland National Forest, CA, beginning one week after the Holy Fire (September 2018) and continuing through January 2020. Soils were extracted for NH4+, NO3-, NO2-, and microbial biomass C and pH was measured.
Results/Conclusions
Burned soils contained high NH4+ concentrations and elevated pH compared to unburned soils. Additionally, NO3-concentrations were elevated in burned soils, suggesting nitrifying communities remained active in burned soils. This is further supported by low, but quantifiable, microbial biomass C measured in burned soils, indicating that either soils were not completely sterilized by fire or nitrifiers have reestablished quickly and begun to metabolize N. These data motivate ongoing work aimed at quantifying the individual contributions of AOA and AOB to soil NO and N2O emissions along a fire intensity gradient using trace gas flux measurements and AOA/AOB inhibition incubations.