Long-Term Nitrogen Additions and Nitrogen Saturation in Two Temperate Forests |
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Authors: | Alison H Magill John D Aber Glenn M Berntson William H McDowell Knute J Nadelhoffer Jerry M Melillo Paul Steudler |
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Institution: | (1) Complex Systems Research Center, University of New Hampshire, Durham, New Hampshire 03824 , US;(2) Department of Natural Resources, University of New Hampshire, Durham, New Hampshire 03824 , US;(3) The Ecosystem Center, Marine Biological Laboratory, Woods Hole, Massachusetts 02354, USA , US |
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Abstract: | This article reports responses of two different forest ecosystems to 9 years (1988–96) of chronic nitrogen (N) additions
at the Harvard Forest, Petersham, Massachusetts. Ammonium nitrate (NH4NO3) was applied to a pine plantation and a native deciduous broad-leaved (hardwood) forest in six equal monthly doses (May–September)
at four rates: control (no fertilizer addition), low N (5 g N m-2 y-1), high N (15 g N m-2
y-1), and low N + sulfur (5 g N m-2 y-1 plus 7.4 g S m-2 y-1). Measurements were made of net N mineralization, net nitrification, N retention, wood production, foliar N content and litter
production, soil C and N content, and concentrations of dissolved organic carbon (DOC) and nitrogen (DON) in soil water. In
the pine stand, nitrate losses were measured after the first year of additions (1989) in the high N plot and increased again
in 1995 and 1996. The hardwood stand showed no significant increases in nitrate leaching until 1995 (high N only), with further
increases in 1996. Overall N retention efficiency (percentage of added N retained) over the 9-year period was 97–100% in the
control and low N plots of both stands, 96% in the hardwood high N plot, and 85% in the pine high N plot. Storage in aboveground
biomass, fine roots, and soil extractable pools accounted for only 16–32% of the added N retained in the amended plots, suggesting
that the one major unmeasured pool, soil organic matter, contains the remaining 68–84%. Short-term redistribution of 15N tracer at natural abundance levels showed similar division between plant and soil pools. Direct measurements of changes
in total soil C and N pools were inconclusive due to high variation in both stands. Woody biomass production increased in
the hardwood high N plot but was significantly reduced in the pine high N plot, relative to controls. A drought-induced increase
in foliar litterfall in the pine stand in 1995 is one possible factor leading to a measured increase in N mineralization,
nitrification, and nitrate loss in the pine high N plot in 1996.
Received 2 April 1999; Accepted 29 October 1999. |
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Keywords: | : ammonium nitrate biomass production foliar chemistry net mineralization net nitrification nitrogen deposition nitrogen saturation soil solution chemistry |
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