Forest Soil Carbon and Nitrogen Losses Associated with Wildfire and Prescribed Fire

Increases in forest wildfires in western North America may affect soil properties. We determined how fire-induced soil C and N losses vary with fuels, thinning, fire type, and burn severity in a southwestern Oregon forest dominated by mature Douglas-fir [Pseudotsuga menziesii var. menziesii (Mirb.) Franco]. Prefire thinning, clear-cutting, and woody debris manipulations on experimental units created a broad range of prefire conditions. Pre- and postfire soil measurements quantified changes in the O horizon plus upper 6.2-cm mineral soil on nine wildfire units, three backburn units, and two prescribed fire units. Fire-induced soil C loss averaged 48% of the initial pool and varied from 3 to 21 Mg C ha(-1). Soil N loss averaged 36% and ranged from 41 to 650 kg N ha(-1). Wildfire-related soil C and N losses were strongly positively related (r(2) = 0.73-0.82) to loss of 1- to 10-cm-diameter woody fuel, which ranged from 4 to 58 Mg ha(-1). Wildfire-related soil C and N losses did not significantly differ between coarse woody debris treatments, but they were twice as great on the thinned than the unthinned treatment. In thinned forest, wildfire resulted in twice as much soil N loss as prescribed fire. Within wildfire and backburn areas, high burn severity, as determined by aboveground measurements, yielded twice the soil C and N losses as lower severity burns. Soil N losses were an order of magnitude larger than estimated N losses from combusted fuels. These comparisons provide unique information linking wildfire-induced soil response to aboveground fuel and forest conditions.