Wildfire Effects on Soil Gross Nitrogen Transformation Rates in Coniferous Forests of Central Idaho, USA

Forest fires often result in a series of biogeochemical processes that increase soil nitrate (NO₃ ⁻) concentrations for several years; however, the dynamic nature of inorganic nitrogen (N) cycling in the plant-microbe-soil complex makes it challenging to determine the direct causes of increased soil NO₃ ⁻. We measured gross inorganic N transformation rates in mineral soils 2 years after wildfires in three central Idaho coniferous forests to determine the causes of the elevated soil NO₃ ⁻. We also measured key factors that could affect the soil N processes, including temperature during soil incubation in situ, soil water content, pH and carbon (C) availability. We found no significant differences (P = 0.461) in gross nitrification rates between burned and control soils. However, microbial NO₃ ⁻ uptake rates were significantly lower (P = 0.078) in burned than control soils. The reduced consumption of NO₃ ⁻ caused slightly elevated NO₃ ⁻ concentrations in the burned soils. C availability was positively correlated with microbial NO₃ ⁻ uptake rates. Despite reduced microbial NO₃ ⁻ uptake capacity in the burned soils, soil microbes were a strong enough N sink to maintain low soil NO₃ ⁻ concentrations 2 years post fire. Soil NH₄ ⁺ concentrations between the treatments were not significantly different (P = 0.673). However, gross NH₄ ⁺ production and microbial uptake rates in burned soils were significantly lower (P = 0.028 and 0.035, respectively) than in the controls, and these rates were positively correlated with C availability. Our results imply that C availability is an important factor regulating soil N cycling of coniferous forests in the region.