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 cause a balance of nitrifying and denitrifying microbial communities in soil, which impacts the release of nitric oxide (NO), nitrous oxide (N2O), and dinitrogen (N2) following a fire. For instance, wildfires can raise soil pH and ammonium (NH4+) levels, leading to an increase in ammonia-oxidizing bacteria (AOB) over archaea (AOA).

Since AOB and AOA handle nitrogen (N) differently, these community shifts can affect N emissions and nitrate (NO3-) availability, potentially impacting denitrifier action and subsequently influencing N emissions over time after a wildfire.

To examine this, researchers inhibited AOB and AOA communities from soil samples collected at four points throughout the year, one year both before and after a chaparral wildfire. By utilizing stable isotopes, they were able to pinpoint the contribution of denitrifier activity to N2O emissions. One year after the wildfire, soil pH rose from 6.1 to 7.0, soil extractable NH4+ levels increased by 30 times, NO3- levels rose by five times, and NO2- levels surged by nine times.

AOB amoA gene copy numbers surged 20 times one year post-fire, while AOA levels stayed constant. Post-fire soil NO emissions skyrocketed 63-fold within a year, with varied contributions from all nitrifying groups. Nitrogen (N)2O emissions peaked eight months after the fire (2271 {+/-} 634 ng N2O-N g-1 soil), with heightened contributions from AOA.

The elevated {delta}15NSPN2O and {delta}15NbulkN2O values indicate increased N2O conversion to N2, indicating that wildfire has elevated AOB abundance versus AOA, fostering nitrification activity and providing denitrifiers with intermediates to increase NO and N2O emissions for up to one year after a wildfire.