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INTRODUCTION
For various reasons, nitrogen (N) has received
more attention in ecology and biogeochemistry
than any other nutrient element. Nitrogen is the
most abundant element in the earth after carbon,
hydrogen, and oxygen and is a major constituent of
amino acids, which are building blocks of proteins,
nucleic acids, and chlorophyll. Nitrogen is found
mainly in the atmosphere (4 x 1021g N), sediment

1997). Nitrogen (N2) comprises 78% of the atmo-
sphere, but this triple-bonded molecule cannot be
used directly by higher plants. Plants rely on rela-
tively small pools of inorganic N (ammonium and
nitrate) and soluble, small molecules of organic N.
Nitrogen commonly limits growth in terrestrial eco-
systems because of large N demands by plants, gen-
erally low inputs of inorganic N, slow turnover of
organic N pools in soils, and losses of N through fire,
leaching, and denitrification.
Nitrogen accumulates in forest soils as a result of
long-term inputs of small quantities of N in precip-
itation, in fallout of particulates, dry deposition of
some N gases, and biological nitrogen fixation. At-
Received 14 December 1999; accepted 22 March 2000.
*Corresponding author; e-mail: dan@cnr.colostate.edu
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