Burn intensity regulates post-fire effects on soil N2O emissions: A grassland soil pyrocosm study

Abstract

Post-fire soil emissions of nitric oxide (NO) and nitrous oxide (N2O) are important ecosystem nitrogen (N) loss pathways, but their flux dynamics vary widely across studies, making generalizations challenging. Natural variation in burn intensity--or heat energy released during combustionm--may help resolve this variability by mediating the soil physicochemical and biological drivers of these emissions. Here, we used an experimental "pyrocosm" system to evaluate how varying burn intensity influences post-fire NO and N2O emissions using soil from a Southern California grassland across 18 weeks. Over several months, we measured soil NO and N2O fluxes alongside soil pH, extractable nitrate (NO3-) and ammonium (NH4+), net NH4+ production, net nitrification, soil carbon (C) pools, microbial biomass C, and the potential activities of five extracellular hydrolytic enzymes under Control, Low- (100.1 {+/-} 20.0 oC peak soil temperature at 5 cm depth), and High-intensity (243.8 {+/-} 7.4 oC peak soil temperature at 5 cm depth) burns. Soil NO emissions were not significantly affected by burn intensity. In contrast, N2O emissions tripled under low burn intensity relative to unburned controls, but did not differ from controls at high burn intensity. This stimulation of N2O emission under low burn intensity was driven by increased denitrification, as supported by elevated soil pH and labile C availability, and an 88% reduction in net nitrification that signaled active NO3- consumption. Under high burn intensity, however, severe loss of microbial biomass, soil C, and NO3- constrained denitrification, with rising hydrolytic enzyme activities over time suggesting C limitation. Together, these findings demonstrate that post-fire N2O emissions reflect a fire intensity-dependent balance between microbial mortality and resource limitation, helping to explain divergent N emissions across ecosystems.