Stephens, E. Z. | bioRxiv Science

bioRxiv2026

Wildfires drive trade-offs in ammonia-oxidizing groups and promote denitrification to increase soil emissions of nitric oxide (NO) and nitrous oxide (N2O) in California chaparral

Wildfires can promote trade-offs in soil nitrifier and denitrifier communities that affect post-fire nitrogen (N) cycling and emissions of nitric oxide (NO), nitrous oxide (N2O), and dinitrogen (N2). For example, by increasing soil pH and ammonium (NH4+), wildfires could increase the abundance of ammonia-oxidizing bacteria (AOB) over archaea (AOA). Because AOB and AOA process N differently, nitrifier trade-offs may affect N emissions and nitrate (NO3-) supply with downstream effects on denitrifier activity, leading us to ask: Do trade-offs between AOA and AOB abundance and shifts in denitrifying processes influence N emissions over time after wildfire? We selectively inhibited AOA and AOB communities from soil collected over four seasonal time points one year before and after a chaparral wildfire and used stable isotopes to parse denitrifier contributions to N2O emissions. Over one year after the wildfire, soil pH increased from 6.1 to 7.0, soil extractable NH4+ increased 30-fold, NO3- increased 5-fold, and NO2- increased 9-fold. AOB amoA gene copy numbers increased 20-fold one year after fire, while AOA abundance remained unchanged. Post-fire soil NO emissions increased 63-fold over one year, with varied contributions from all nitrifier groups. Soil N2O emissions peaked eight months after fire (2271 {+/-} 634 ng N2O-N g-1 soil), with increased contributions from AOA. Wildfire increased {delta}15NSPN2O and {delta}15NbulkN2O values, suggesting increased N2O reduction to N2. Overall, wildfire increased AOB abundance relative to AOA, promoting nitrification activity and providing intermediates to denitrifiers to increase emissions of NO and N2O for up to one year after fire.

ecology↗

bioRxiv2025

Wildfire alters nitrogen cycling to increase soil emissions of nitric oxide (NO) and the heterogeneity of nitrous oxide (N2O) in California chaparral

Wildfires can disrupt ecosystem nitrogen (N) cycling by combusting vegetation biomass N and depositing ash that is rich in ammonium (NH4+) onto soils. Post-fire increases in NH4+ and soil physicochemical changes may promote further N loss by stimulating microbially-driven emissions of nitric oxide (NO) and nitrous oxide (N2O)--trace gases that alter air quality and climate. We hypothesized that soil NO and N2O emissions would increase with the flush of post-fire N availability and would be highest in soils that burned at medium severity due to the combination of high soil N availability and persistence of microbial activity. To test this, we established nine plots (6 burned; 3 unburned) and sampled soils seasonally over three years after wildfire in a chaparral shrubland in Southern California, USA. Wildfire significantly increased soil extractable NH4+ by an average of 10 {micro}g NH4+-N g soil-1, soil extractable NO3- by 8 {micro}g NO3--N, and soil pH by 0.5 units. Post-fire soil NO emissions significantly increased by an average 74 ng NO-N g-1 (cumulative 40-h incubations) over three years, with the highest emissions measured in year one from plots that burned at medium and high severities. No significant effects of burning were detectable for N2O emissions over three years (average {+/-} standard error; 224 {+/-} 107 ng N2O-N g-1 soil in burned plots and 30 {+/-} 17 ng N2O-N g-1 soil in unburned plots); however, we observed high fluxes only from soils that burned at medium and high severities (N2O > 3500 ng N2O-N g-1 soil). Isotopic characterization of N2O from high-emitting soils indicated contributions from diverse sources and increased N2O reduction to N2. Overall, the occurrence of high N2O fluxes and accelerated NO emissions post fire indicate wildfires interact with soil N cycling to promote burn-severity-sensitive gaseous N losses long after wildfires are extinguished.

ecology↗