OOS 19-2 - Wildfire alters nitrifier communities and soil emissions of NOx and N2O in California chaparral

Fires can drive ecosystem N loss by volatilizing N bound in plant biomass to the atmosphere and by leaving behind ash rich in ammonium (NH4+) and organic N that can run off when it rains. While N volatilization and runoff account for a large fraction of N loss after fires, budget imbalances suggest soil emissions of nitric oxide (NO) and nitrous oxide (N2O) may also be significant N loss pathways after fire. Soil emissions of NO and N2O are largely governed by the microbial processes of nitrification and denitrification. Under aerobic conditions typical of dry soils, nitrifying organisms such as ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) oxidize NH4+ to nitrate (NO3-) and release NO and N2O as byproducts. AOA and AOB process N with different efficiencies; specifically, AOB dominate in high NH4+ and pH environments and produce higher NO and N2O emissions. As such conditions are frequently observed after fires, we hypothesize NO and N2O emissions will increase as AOB communities become dominant. To test this, we collected soil cores from 5 plots in Sequoia National Park, CA both one month before a high severity chaparral fire and seasonally for one year after. We selectively inhibited AOA and AOB communities to measure their contributions to NO and N2O emissions. We also measured the isotopic composition of N2O emissions from these soils using an LGR isotopic N2O analyzer to better understand the processes responsible for post-fire N2O production. One month after the fire, soil bulk emissions of NO over 72 h were 1.5 times higher in the burned plots (101.4 ± 22.4 µg N-NO/g soil burned; 67.1 ± 19.3 µg N-NO/g soil unburned; ± standard error (SE)). Bulk soil emissions of N2O over 72 h were 7.5 times lower in burned plots compared to before the fire (0.06 ± 0.04 ng N-N2O/g soil burned; 0.5 ± 0.2 ng N-N2O/g soil unburned; ±SE). Although at one month post-fire nitrifier contributions to NO emissions were not significantly different compared to pre-burn (AOA: p=0.09, AOB: p=0.162), overall NO produced by AOA and AOB increased one month post fire. Results for post-fire nitrifier contributions to N2O emissions were highly variable and non-significant with no clear trends as all N2O emissions were near zero. Pairing these data with isotopic information and further analysis over time post-fire (in progress) may yield one of the most in-depth understandings of post-fire NO and N2O emissions to date.