CARBON AND NITROGEN IN THE LIGHT FRACTION OF A FOREST SOIL

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Abstract
We extracted light-fraction material densimetrically in Nal solution (specific gravity 1.65 g/cm3) from root-free soil samples from a forested site in the Oregon Cascade Mountains. We chose extraction at this density because it excluded most of the organomineral particles from the light fraction. Major components identifiable from scanning electron micrographs were dead root fragments, hyphae, charcoal, and pumice–all with adsorbed or entrapped colloidal particles. Total soil carbon (C) and nitrogen (N) at the site averaged 145 and 7.4 megagrams per hectare, respectively, to a depth of 83 centimeters. Light-fraction material accounted for 53 percent of the total C and 45 percent of the total N in the 0− to 3-cm layer; these proportions decreased abruptly in the next layer and then more gradually over the remainder of the 83-cm profile. Amounts of light-fraction material increased by 50 to 100 percent from early spring to summer and then peaked again in the fall. This large seasonal fluctuation indicates that the light fraction provides an important labile reservoir of C and nutrient elements in the forest ecosystem. We extracted light-fraction material densimetrically in Nal solution (specific gravity 1.65 g/cm3) from root-free soil samples from a forested site in the Oregon Cascade Mountains. We chose extraction at this density because it excluded most of the organomineral particles from the light fraction. Major components identifiable from scanning electron micrographs were dead root fragments, hyphae, charcoal, and pumice–all with adsorbed or entrapped colloidal particles. Total soil carbon (C) and nitrogen (N) at the site averaged 145 and 7.4 megagrams per hectare, respectively, to a depth of 83 centimeters. Light-fraction material accounted for 53 percent of the total C and 45 percent of the total N in the 0− to 3-cm layer; these proportions decreased abruptly in the next layer and then more gradually over the remainder of the 83-cm profile. Amounts of light-fraction material increased by 50 to 100 percent from early spring to summer and then peaked again in the fall. This large seasonal fluctuation indicates that the light fraction provides an important labile reservoir of C and nutrient elements in the forest ecosystem. © Williams & Wilkins 1983. All Rights Reserved.