Fire changes forests both above ground and below ground. Severe fire effects on soils include combusting some (or all) of the forest floor, reducing soil carbon (C) and nutrient contents, and altering many soil properties. Changes in soil C and nutrients, especially nitrogen (N), may be especially important factors that influence stand recovery rates and biotic response after fire. Fire disturbances in forests have been studied most often in terms of above-ground effects, with less attention given to below-ground effects, and usually have been limited to understanding short-term effects. In contrast, studies of the long-term effects of fire on soils in temperate coniferous forest ecosystems are exceedingly rare. Consequently, theories of long-term fire effects on soils are based primarily on extrapolation from short-term observations and experiments.
Changes in soil nutrient capital and dynamics following fire result primarily from organic matter combustion and from soil heating. Although fire effects on soils can be extremely patchy, there typically is a net loss of C and N from soils when averaged over fire-affected areas. Losses of C–essentially losses of soil organic matter–reduce soil structure and moisture-holding capacity. Fire-induced losses of N from organic matter combustion occur in concert with C losses, and further N may be lost from soils as post-fire stands regrow and transfer N from soils to above-ground vegetation. Whether such losses persist over long periods remains unknown, yet has important implications for understanding how changes in fire regimes due to climate, suppression or both will affect long-term sustainability of forest productivity and other processes in fire-dominated landscapes. To address this, we examined how time-since-fire influences forest floor and surface soil pools of C and N in natural unlogged stands of the western Cascades of Oregon.
