Is there synchronicity in nitrogen input and output fluxes at the Noland Divide Watershed, a small N-saturated forested catchment in the Great Smoky Mountains National Park?
High-elevation red spruce [Picea rubens Sarg.]-Fraser fir [Abies fraseri (Pursh.) Poir] forests in the Southern Appalachians currently receive large nitrogen (N) inputs via atmospheric deposition (30 kg N ha(-1) year(-1)) but have limited N retention capacity due to a combination of stand age, heavy fir mortality caused by exotic insect infestations, and numerous gaps caused by windfalls and ice storms. This study examined the magnitude and timing of the N fluxes into, through, and out of a small, first-order catchment in the Great Smoky Mountains National Park. It also examined the role of climatic conditions in causing interannual variations in the N output signal. About half of the atmospheric N input was exported annually in the streamwater, primarily as nitrate (NO3-N). While most incoming ammonium (NH4-N) was retained in the canopy and the forest floor, the NO3-N fluxes were very dynamic in space as well as in time. There was a clear decoupling between NO3-N input and output fluxes. Atmospheric N input was greatest in the growing season while largest NO3-N losses typically occurred in the dormant season. Also, as water passed through the various catchment compartments, the NO3-N flux declined below the canopy, increased in the upper soil due to internal N mineralization and nitrification, and declined again deeper in the mineral soil due to plant uptake and microbial processing. Temperature control on N production and hydrologic control on NO3-N leaching during the growing season likely caused the observed inter-annual variation in fall peak NO3-N concentrations and N discharge rates in the stream.
Van Miegroet
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Department of Forest Resources, Utah State University, Logan 84322-5215, USA. helgavm@cc.usu.edu
Creed
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Nicholas
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Tarboton
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DG
Webster
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Shubzda
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J
Robinson
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B
Smoot
J
J
Johnson
D W
DW
Lindberg
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SE
Lovett
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G
Nodvin
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S
Moore
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D000531:Altitude; D000641:Ammonia / Q000032:analysis; D017753:Ecosystem; D005618:Fresh Water / Q000737:chemistry*; D005843:Geography; D009566:Nitrates / Q000032:analysis; D009584:Nitrogen / Q000032:analysis* / Q000378:metabolism; D009657:North Carolina; D012621:Seasons; D012987:Soil / Q000032:analysis; D013696:Temperature; D013714:Tennessee; D014197:Trees / Q000737:chemistry / Q000378:metabolism
D009566:Nitrates; D012987:Soil; D000641:Ammonia; D009584:Nitrogen