Changes in Canopy Processes Following Whole-Forest Canopy Nitrogen Fertilization of a Mature Spruce-Hemlock Forest |
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Authors: | E Gaige D B Dail D Y Hollinger E A Davidson I J Fernandez H Sievering A White W Halteman |
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Institution: | (1) Department of Plant, Soil, and Environmental Sciences, University of Maine, 5722 Deering Hall, Orono, Maine 04469-5722, USA;(2) US Forest Service, Northern Region Research Station, 271 Mast Road, Durham, New Hampshire 03824, USA;(3) Woods Hole Research Center, 149 Woods Hole Road, Falmouth, Massachusetts 02540-1644, USA;(4) Geography and Environmental Sciences, University of Colorado-Denver, Campus Box 172, P.O. Box 173364, Denver, Colorado 80217-3364, USA;(5) Forest Ecosystem Science, School of Forest Resources, University of Maine, 5755 Nutting Hall, Orono, Maine 04469-5755, USA;(6) Math and Statistics, University of Maine, 5752 Neville Hall, Orono, Maine 04469-5752, USA |
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Abstract: | Abstract
Most experimental additions of nitrogen to forest ecosystems apply the N to the forest floor, bypassing important processes
taking place in the canopy, including canopy retention of N and/or conversion of N from one form to another. To quantify these
processes, we carried out a large-scale experiment and determined the fate of nitrogen applied directly to a mature coniferous
forest canopy in central Maine (18–20 kg N ha−1 y−1 as NH4NO3 applied as a mist using a helicopter). In 2003 and 2004 we measured NO3
−, NH4
+, and total dissolved N (TDN) in canopy throughfall (TF) and stemflow (SF) events after each of two growing season applications.
Dissolved organic N (DON) was greater than 80% of the TDN under ambient inputs; however NO3
− accounted for more than 50% of TF N in the treated plots, followed by NH4
+ (35%) and DON (15%). Although NO3
− was slightly more efficiently retained by the canopy under ambient inputs, canopy retention of NH4
+as a percent of inputs increased markedly under fertilization. Recovery of less than 30% of the fertilizer N in TF suggested
that the forest canopy retained more than 70% of the applied N (>80% when corrected for N which bypassed tree surfaces at
the time of fertilizer addition). Results from plots receiving 15N enriched NO3
− and NH4
+ confirmed bulk N estimations that more NO3
− than NH4
+ was washed from the canopy by wet deposition. The isotope data did not show evidence of canopy nitrification, as has been
reported in other spruce forests receiving much higher N inputs. Conversions of fertilizer-N to DON were observed in TF for
both 15NH4
+ and 15NO3
− additions, and occurred within days of the application. Subsequent rain events were not significantly enriched in 15N, suggesting that canopy DON formation was a rapid process related to recent N inputs to the canopy. We speculate that DON
may arise from lichen and/or microbial N cycling rather than assimilation and re-release by tree tissues in this forest. Canopy
retention of experimentally added N may meet and exceed calculated annual forest tree demand, although we do not know what
fraction of retained N was actually physiologically assimilated by the plants. The observed retention and transformation of
DIN within the canopy demonstrate that the fate and ecosystem consequences of N inputs from atmospheric deposition are likely
influenced by forest canopy processes, which should be considered in N addition studies.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. |
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Keywords: | nitrogen deposition canopy fertilization canopy N retention throughfall stemflow 15N tracer |
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