Carbon and nitrogen in the light fraction of a forest soil: vertical distribution and seasonal patterns

We extracted light-fraction material densimetrically in NaI solution (specific gravity1.65 g/cm3) from root-free soil samples from a forested site in the Oregon CascadeMountains. We chose extraction at this density because it excluded most of theorganomineral particles from the light fraction. Major components identifiable fromscanning electron micrographs were dead root fragments, hyphae, charcoal, and pum-ice—all with adsorbed or entrapped colloidal particles. Total soil carbon (C) andnitrogen (N) at the site averaged 145 and 7.4 megagrams per hectare, respectively, to adepth of 83 centimeters. Light-fraction material accounted for 53 percent of the total Cand 45 percent of the total N in the 0- to 3-cm layer; these proportions decreasedabruptly in the next layer and then more gradually over the remainder of the 83-cmprofile. Amounts of light-fraction material increased by 50 to 100 percent from earlyspring to summer and then peaked again in the fall. This large seasonal fluctuationindicates that the light fraction provides an important labile reservoir of C and nutrientelements in the forest ecosystem.