Wildfires alter nitrifier communities and increase soil emissions of NOx but not N2O in California chaparral

Background:

Fires burn roughly 3% of Earth’s land surface each year and are predicted to become more frequent and severe as human-caused climate change progresses. 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. Identifying sources of NO and N₂O is important because NO is a precursor for tropospheric O₃ which causes high rates of asthma hospitalizations,and N₂O is a powerful greenhouse gas with 300× the warming potential of CO₂. 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, suggesting shifts in AOA:AOB ratios may change N emissions. Specifically, AOB are dominant in soils with high NH₄⁺and pH and produce higher NO and N₂O emissions. Since such soil 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 the Sequoia National Park, CA over a time series starting two weeks after a high severity chaparral fire. 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.

Results/Conclusions

One month after the fire, soil bulk emissions of NO over 72hrs 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; ±SE). Bulk soil emissions of N₂O over 72hrs were 7.5 times lower in burned plots compared to before the fire (0.0616 ± 0.04 ng N-N₂O/g soil burned; 0.463 ± 0.19 ng N-N₂O/g soil unburned; ±SE). Although the effects of fire on nitrifier communities were not significant at one month post-fire (Control: p=0.14, AOA: p=0.09, AOB: p=0.162), both AOA and AOB contributions to NO emissions increased in response to fire. Results for nitrifier contributions to N₂O emissions were highly variable and non-significant with no clear trends as all N₂O emissions were near zero. Further analysis over the time series may yield clearer results as microbial communities have more time to recover. Pairing these data with isotopic information (in progress) may yield one of the most in-depth understandings of post-fire NO and N₂O emissions to date.