Soil carbon pools and fluxes vary across a burn severity gradient three years after wildfire in Sierra Nevada mixed-conifer forest

Carbon (C) storage in soils contributes to the strength and stability of total ecosystem C sinks, but both aboveground and belowground C is vulnerable to loss during fire. The distribution of soil C and nitrogen (N) among various defined pools - e.g., active, slow and resistant C, and ammonium and nitrate as forms of inorganic N determines the C storage capacity of forests and the nutrient availability for plant communities recovering from wildfires. Projections of increased wildfire severity due to a warming climate and frequent droughts raise concerns about parallel increases in fire's impacts on the sizes and mineralization kinetics of soil C and N pools, with potentially long-lasting effects on the strength of the forest C sink and on the ability of forests to recover from disturbance. Therefore, we sought to determine how the sizes and mineralization rates of soil C and N pools vary across a gradient of fire severity three years after the Chips Fire burned 30,500 ha of Sierra Nevada mixed conifer forest. We measured total C and N in forest floor and mineral soil (0-5 cm), the pool sizes and mean residence times of the active, slow, and resistant C in mineral soil, and the pool sizes and mineralization rates of inorganic N in mineral soil. Forest floor total C was lower in areas that experienced high severity fire than in unburned reference areas, an effect likely attributable to greater combustion of forest floor material in high severity areas. Mineral soil C content did not vary with fire severity. Over a 300-day lab incubation, mineral soil CO2-C efflux rates were consistently lower in soils from areas that experienced high severity fire relative to unburned reference areas and were associated with longer mean residence times of the slow C pool. Forest floor N content was lower in high severity areas than unburned areas, whereas mineral soil total N did not vary with fire severity. Mineral soil ammonium and total inorganic N concentrations increased significantly with fire severity in field-fresh soils, but this trend was no longer apparent after a 300-day lab incubation, indicating that site-specific factors control N availability among fire severity levels. Our results indicate that future increases in wildfire severity in mixed-conifer forest may alter the strength of the forest C sink by impacting the amount C stored in forest floor, the stability of mineral soil C, and the availability of N to recovering plant communities.