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?

H Van Miegroet,D W Johnson,J Smoot,D.G Tarboton,B Robinson,S Nodvin,I.F Creed,S E Lindberg,S Moore,N.S Nicholas,K.L Webster,G Lovett,J Shubzda

doi:10.1100/tsw.2001.384

H Van Miegroet, D W Johnson + Show 11 more

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https://doi.org/10.1100/tsw.2001.384

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Journal: TheScientificWorldJournal	Publication Date: Jan 1, 2001
Citations: 30	License type: CC BY 3.0

Affiliation: Utah State University

Abstract

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.

Highlights

Based on the simple atmospheric input-streamwater output Nmass balance presented in Table 1, approximately 50% the incoming N was retained at Noland Divide Watershed (NDW)
While the annual N budget would suggest that this catchment was a simple flow-through system especially for NO3-N, a somewhat different picture emerged when examining the changes in N fluxes as the solution passed through the various components of the watershed (Table 2)
While NH4-N accounted for a significant portion of the atmospheric N input flux, most of this inorganic N form was effectively retained in the forest canopy and litter layer

Summary

Objectives

The objective of this paper is to use currently available water chemistry data that have been collected since 1986 at the NDW and at an adjacent intensive biogeochemical research plot to (1) assist us in better understanding N biogeochemistry in this highelevation forest, (2) establish the magnitude and timing of the variation in solution chemistry, and (3) identify broad drivers for

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Conclusion