PARENT SESSION
Poster Session # 14: Forest Ecology.

Wednesday, August 6 Presentation from 5:00 PM to 6:30 PM. SITCC Exhibit Hall B.

Carbon and nitrogen pools in old- and second-growth Pacific Northwest coniferous forests.

Barry, Matthew^*,1, Klopatek, Jeffrey¹, ¹ Arizona State University, Tempe, Arizona

ABSTRACT- Pacific Northwest (PNW) coniferous forests contain the greatest amount of belowground carbon (C) storage and flux of any ecosystem in the conterminous United States. Nitrogen is a regulator of C sequestration and release in these forests. We compared inorganic N and belowground C and N pools in three recently logged (young; 25 y) and three old-growth (450-750 y) forest stands in south-central Washington at and near the Wind River Canopy Crane Research Facility. Total ammonium (NH₄) and nitrate (NO₃) levels were quantified along a vertical gradient throughout the forest stands using ion exchange resin lysimeters. Lysimeters were placed at 10 m intervals descending the canopy (in three different tree species), above and below the forest floor, and 20 cm into the mineral soil. We also measured C and N pools in forest floor material and soils to a depth of one meter. Forest canopies absorbed a substantial proportion of inorganic N, and our results are compared to data from a local NADP monitoring site. Ammonium levels exceeded NO₃ in all stands, and the old-growth stands tended to have more NH₄ in soils than young. Within age class, sites with red alder (a N-fixing species) exhibited more NH₄ in soils than those without N-fixers. In contrast to NO₃, ammonium patterns mimicked N pools for all stands and were maintained throughout the belowground profile. Carbon:nitrogen ratios increased with age in forest floor material due to both higher C and lower N concentrations in old-growth compared to second growth stands. However, C:N ratios decreased with stand age in mineral soils. Total forest floor C and N pools were larger in the old-growth stands due to greater biomass. Carbon and N pools were also greater in old-growth soils at 0-10 cm depths but no differences were observed in deeper soils.

Key words: Nitrogen, Pacific Northwest, Carbon, Coniferous Forest