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

Abstract Wildfires can disrupt ecosystem nitrogen (N) cycling by combusting vegetation biomass N and depositing ash that is rich in ammonium (NH 4 + ) onto soils. Post-fire increases in NH 4 + and soil physicochemical changes may promote further N loss by stimulating microbially-driven emissions of nitric oxide (NO) and nitrous oxide (N 2 O)—trace gases that alter air quality and climate. We hypothesized that soil NO and N 2 O 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 NH 4 + by an average of 10 µg NH 4 + -N g soil -1 , soil extractable NO 3 - by 8 µg NO 3 - -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 N 2 O emissions over three years (average ± standard error; 224 ± 107 ng N 2 O-N g -1 soil in burned plots and 30 ± 17 ng N 2 O-N g -1 soil in unburned plots); however, we observed high fluxes only from soils that burned at medium and high severities (N 2 O > 3500 ng N 2 O-N g -1 soil). Isotopic characterization of N 2 O from high-emitting soils indicated contributions from diverse sources and increased N 2 O reduction to N 2 . Overall, the occurrence of high N 2 O 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.