Intense forest wildfire sharply reduces mineral soil C and N: the first direct evidence

Direct evidence of the effects of intense wildfire on forest soil is rare because reliable prefire data are lacking. By chance, an established large-scale experiment was partially burned in the 2002 Biscuit fire in southwestern Oregon. About 200 grid points were sampled across seven burned and seven unburned stands before and after the fire. Fire-related soil changes - including losses of soil organic and inorganic matter - were so large that they became complicated to measure. The 51 Mg.ha(-1) of loose rocks on the soil surface after fire suggests erosion of 127 Mg.ha(-1) of fine mineral soil, some of which likely left in the fire plume. After accounting for structural changes and erosion with a comparable-layers approach, combined losses from the O horizon and mineral soil totaled 23 Mg C.ha(-1) and 690 kg N.ha(-1), of which 60% (C) and 57% (N) were lost from mineral horizons. Applying a fixed-depth calculation - commonly used in previous fire studies - that disregards structural changes and erosion led to underestimates of loss of nearly 50% for C and 25% for N. Although recent debate has centered on the effects of postwildfire forest management on wood, wildlife habitat, and fuels, this study indicates that more consideration should be given to the possible release of greenhouse gases and reduction of future forest productivity and CO2 uptake.