Fire effects on gross inorganic N transformation in riparian soils in coniferous forests of central Idaho, USA: wildfires v. prescribed fires

We investigated differences between wildfires and prescribed fires in their effects on nitrogen (N) dynamics in mineral soils collected from riparian coniferous forests of central Idaho, USA. Specifically, we investigated how the two types of fires affected inorganic N concentrations, microbial biomass N and gross transformation rates of inorganic N in mineral soils relative to their corresponding unburnt controls. There was no significant difference in soil NH4+ concentrations between burnt and control soils in either type of fires. However, wildfires significantly reduced gross ammonification and microbial NH4+ uptake rates relative to their controls (P = 0.05 and 0.08). No such effect was found in soils burnt by the prescribed fires relative to their controls. Burnt soils had significantly higher NO3– concentrations than control soils when all the data were pooled (P = 0.08). The elevated NO3– concentrations in the soils burnt by either type of fire were not caused by increased gross nitrification, but likely by significantly reduced microbial NO3– uptake (P ≤ 0.02). We concluded that controlled prescribed fires conducted in early spring had less of an effect on soil N dynamics than wildfires in the region.