Oral Session # 98: Biogeochemistry V: Forests; Nitrogen.
Presiding: RL Sanford
Friday, August 8. 8:30 AM to 12:00 PM, SITCC Meeting Room 204.

Fertilizer uptake by riparian woody vegetation: A 15N tracer experiment.

Chambers, Chris*,1, Marshall, John1, Danehy, Bob2, 1 University of Idaho, Moscow, ID2 Boise Cascade Corporation, Boise, ID

ABSTRACT- The nutrient balance of streams and adjacent riparian ecosystems in the Pacific Northwest has been influenced by the elimination of anadromous fish runs and perhaps by forest fertilization. To improve our understanding of nutrient cycling on land, spiraling in the stream channel, and the nutient linkage between the two ecosystems, we fertilized two streams in central Idaho. Two levels of nitrogen dose (224kgN/ha and 448kgN/ha) were applied on each stream. The fertilizer mix included a 15N-enriched tracer. This presentation focuses on fertilizer uptake, expressed as percent nitrogen derived from fertilizer (%Ndff), in the terrestrial woody plant community. Pre-treatment 15N differed between the two creeks, most likely due to residual salmon nitrogen on one of the creeks, and this difference was still detectable following treatment (p < .001). Although the two creeks differed in pre-treatment 15N, %Ndff did not differ significantly between creeks, nor was the interaction between creek and treatment significant (p = .15 and .13 respectively). The two dominant tree species on both sites, Abies lasiocarpa and Picea engelmannii, displayed no significant difference in 15N or %Ndff (difference in means: 15N = .39‰ ± .73 (se), %Ndff = 3.23% ± 5.9). Most of the fertilizer taken up was allocated to current year needles in conifers. 224kgN/ha and 448kgN/ha treatments responded similarly in uptake of fertilizer N (%Ndff parameter estimates = 6.1% ± 6.9, 5.9% ± 5.1 respectively). Alnus incana, the only nitrogen-fixing shrub in the sample, took up a much smaller proportion of fertilizer than did other deciduous shrubs (mean %Ndff of shrubs = 29.0% ± 2.6, A. incana = 4.57% ± 5.12). Marginal enrichment in conifer litter (mean %Ndff = 6.2% ± 3.3) was evident, but fertilizer N made up 25% (± 4.2%) of total deciduous shrub litter N. Thus fertilizer N appeared disproportionately in the shrubs and in the new needles of the streamside conifers. Future work will trace this label into litterfall and into the streams.

Key words: nitrogen cycling, 15N label, riparian fertilization