Barriga, M. F. P. | bioRxiv Science

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↗

bioRxiv2025

Mojave Desert microbial communities show high resistance and resilience over three years despite widespread plant mortality following the Dome Fire

O_LIHigh severity desert fires are uncommon but typically chart a new successional trajectory altering plant communities for at least 65 years. These aboveground vegetation shifts can have large implications for belowground microbial communities that maintain soil structure and nutrient cycling. High severity wildfires in forests or shrublands can severely reduce microbial species richness and biomass and alter microbiomes for decades but impacts on desert soil microbiomes are virtually unknown. C_LIO_LIThe 2020 Mojave Desert Dome Fire burned 43,273 acres of Eastern Joshua tree (Yucca jaegeriana) habitat, burning roughly 1 million trees. To track aboveground and belowground impacts of the Dome Fire, we established 9 plots (6 burned; 3 unburned) and sampled 4 subsamples per plot for 5 time points ranging from 2 weeks to 3 years post-fire. We measured initial ash depth as a proxy of soil burn severity and assessed plant mortality, plant richness, soil chemical characteristics, estimated soil microbial biomass with qPCR, and microbial richness and composition with Illumina MiSeq of 16S and ITS2 amplicons. C_LIO_LIBelowground communities were highly diverse, containing 25,444 bacterial, 269 archaeal, and 6,683 fungal ASVs amplicon sequence variants (ASVs) or microbial taxa. We identified at least 65 plant species and saw 80% Eastern Joshua tree mortality in burned plots over three years, with reduced plant richness post-fire except an abundance of annual herbs at 1-year post-fire, yet the fire did not significantly reduce microbial biomass or richness at any time point. C_LIO_LIMicrobial communities for both bacteria and fungi showed small but significant changes, enriching for pyrophilous microbes in burned plots. We identified increases of pyrophilous microbes such as Tumebacillus, Massilia, Noviherbaspirillum bacteria and Pseudotricharina, Penicillium, Coniochaeta and Naganishia fungi. C_LIO_LISynthesis: We present the first comprehensive above and belowground examination following a natural desert wildfire including Archaea, Bacteria, and Fungi. Despite the widespread mortality of Eastern Joshua trees across 3 years, microbial biomass, richness, and community composition were mostly resistant to change, like microbial responses to low-intensity fast-moving grassland fires. Despite high resistance overall, wildfire still increased several pyrophilous bacterial and fungal taxa common after high severity shrubland and forest wildfires. C_LI

microbiology↗