Date of Award

8-2005

Level of Access Assigned by Author

Campus-Only Thesis

Degree Name

Master of Science (MS)

Department

Ecology and Environmental Sciences

Advisor

David Bryan Dail

Second Committee Member

Ivan J. Fernandez

Third Committee Member

Alan S. White

Abstract

Nitrogen (N) deposition has the potential to cause ecosystem decline as a result of N saturation or increased carbon sequestration as a result of relieving N limitation in forested ecosystems. The Howland Integrated Forest Study (HIFS) site uses 5 helicopter flights per growing season to apply 18 kg N ha-1 yr-1 as NH4NO3 to a 21 ha area of spruce-hemlock forest. Within this area are 3 intensively sampled subplots and an adjacent plot serves as a control. Fertilization began in 2001 and continues through 2005. In 2001 through 2003, two of the subplots received 15N label, either as NO3- or NH4+ at an additional 1.8 kg N ha-1 yr-l. Precipitation events yielded 13 throughfall (TF) samples in 2003 and 15 samples in 2004, unevenly spaced throughout the growing season and around the fertilization events. Samples were analyzed for NO3M-, NH4+ and DON concentration and fluxes were calculated for each TF sample. A multivariate repeated measures analysis of variance was used to determine differences in flux of N over the growing season.

Data from 2003 and 2004 show that bulk TF N flux was dramatically changed from ambient by the fertilization regime. While dissolved organic N (DON) comprised greater than 80 percent of the N in TF under ambient inputs, NO3- accounted for greater than 50 percent of TF N in the treated plots. Although NO3- was more efficiently removed from the canopy than NH4+ (if any preference could be determined) under ambient conditions, a dramatic preference was observed for NH4+ under the fertilization regime. Data from 2003 when 15N was being added to two of the subplots shows that more 15NO3- was washed from the canopy and collected in TF, confirming evidence in the bulk TF data of a preferential retention of NH4+ by the canopy. The 15N data did not show evidence of canopy nitrification, the oxidation of NH4+ to NO3-, as added 15NO3- was not recovered as 15NO3- at a significant level. Although no statistically significant treatment effects were found when DON fluxes were analyzed over the growing season, short-term transformation of 15NO3- to DO15N was statistically significant. Although DON enrichment was also observed where 15NO4+ was applied to the canopy, this was not statistically significant.

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