Prescribed biomass burning effects on soil NO emissions in South Carolina.

Nitrogen oxides (NOx = NO + NO2) are key atmospheric trace gases that affect air quality, oxidation chemistry, and nitrogen deposition. Although anthropogenic NOx emissions have declined due to regulations, natural sources like soil emissions are becoming relatively more important. Biomass burning can alter soil nitrogen cycling and potentially influence soil NO fluxes, yet few controlled studies exist. Here, we examined how prescribed burning affects soil NO emissions in a South Carolina forest using incubation experiments. The study site is a sandhill ecosystem dominated by upland oak and longleaf pine, with fire-adapted understory vegetation including wiregrass. Soil cores were collected before and after burns of varying intensities. Measured soil pH, nitrate, and ammonium showed no consistent differences between pre- and post-burn conditions. NO fluxes, quantified using a custom soil chamber, averaged 0.056 ± 0.068 ng-N h-1 g-1 with no significant burn-related change, and most values remained below the method detection limit of 0.137 ng-N h-1 g-1. However, ammonium nitrate additions triggered significant NO emissions, highlighting nitrogen availability as a key control. Scaling fluxes to the state level, we estimate natural forest soil contributed an estimated 700 ± 800 metric tons of NO during a summer month, which is about 5.0 ± 6.0% of statewide NO emissions. These results suggest that prescribed fire does not enhance soil NO emissions in nitrogen-limited, acidic southeastern U.S. forests, immediately following fire events. Moreover, they indicate that forest soil emissions are likely a minor contributor to regional NO budgets in this region.