PARENT SESSION
Oral Session #52: Carbon and Nitrogen Cycling: Soil processes and patterns. Presiding: F. Gilliam.
Wednesday, August 8, 2001. 1:00 PM to 5:00 PM. Hall of Ideas E.

The fate and retention of organic and inorganic nitrogen in a western Oregon coniferous forest .

Holub, Scott¹, Lajtha, Kate¹, ¹

ABSTRACT- Organic nitrogen from the N₂-fixing epiphytic lichen, Lobaria oregana, contributes up to 80% of new N inputs in Pacific Northwest old-growth forests. Our objective was to determine the ecosystem pools involved in the retention of complex organic N inputs and to compare those results with inorganic N over the course of one year in a mid-elevation old-growth forest at the H.J. Andrews LTER. We added ¹⁵N-labeled Lobaria, bread yeast, tannin-complexed yeast, and NH₄Cl separately to large in situ soil cores. Upon collection, each core was separated into understory plant, moss, litter/O horizon, 0-5 cm soil, and 5-15 cm soil pools. All pools were analyzed for total ¹⁵N. Litter and 0-5 cm soil pools were also analyzed for inorganic N, DON, and microbial biomass N. Selected 0-5 cm soils were separated using density fractionation. Total ¹⁵N recovery over all dates was not significantly lower than 100%. For all treatments, the litter/O horizon was the largest sink for the added N, although the mineral soil horizons tended to increase as a sink through time. Little of the total ¹⁵N was found as inorganic N. The light and heavy density fractions showed similar ¹⁵N labeling, which indicated that the sink strength per N molecule was similar in both fractions. Our results indicate that the soil pools at our site at H.J. Andrews have substantial ability to retain organic and inorganic N forms.

KEY WORDS: Organic Nitrogen, Retention, Forest Soil, Lobaria oregana