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mospheric deposition includes ammonium and ni-
trate salts and nitric acid. Nitrogen fixation is the
reduction of N2 to NH3, followed by amination of
ammonium for incorporation into a suite of bi-
omolecules. Common rates of nitrogen deposition
from the atmosphere range from 1 to 5 kg N ha-1
y-1
in areas without industrial pollution to more than
25 kg N ha-1
y-1
for areas of high dairy and industry
development in Europe. Deposition of N gases is
probably slight, with the exception of significant
deposition of ammonia and nitric acid vapor in
polluted areas with high atmospheric concentra-
tions. Only relatively few strains of bacteria can fix
N, including free-living aerobic and anaerobic sin-
gle-celled bacteria, photosynthetic cyanobacteria,
and some strains that live in symbiotic associations
in lichens and in higher plants. Free-living N fixa-
tion generally is thought to be constrained by en-
vironmental conditions and low supplies of carbon
substrates, with rates usually less than a few kg N
ha-1
y-1
(Son 2000).
This general picture of the N inputs to forest soils
builds on 2 centuries of research in chemistry, mi-
crobiology, plant physiology, and atmospheric sci-
ences. Is it likely that any major surprises remain
undiscovered in the N cycle of forests? This ques-
tion may be partially answered by listing some sur-
prises from just the past decade including:
c very high rate of gross N mineralization (release)
in soils compared with the net quantities of N
that remain after microbial immobilization (see
Kaye and Hart 1997)
c high rates of nitrate production (gross nitrifica-
tion) in most forests, even N-poor conifer sys-
tems, with high rates of nitrate uptake by mi-
crobes yielding low net accumulation of nitrate
(net nitrification; see Stark and Hart 1997)
c substantial nitrification by heterotrophic mi-
crobes in some acidic forest soils (see Klingen-
smith and Van Cleve 1993; Hart and others
1997)
c generation of nitrous oxide (N2O) from nitrifi-
cation processes in excess of N2O production in
denitrification processes (see Papen and Butter-
bach-Bahl 1999).
How well do we understand the rates of N inputs to
forests? The state-of-the-science of estimating at-
mospheric deposition rates is thought to have a
precision of approximately 630% for well-charac-
terized low elevation forests, or 650% for high-
elevation forests with substantial fog inputs (Lovett
1992). Rates of N input from nonsymbiotic N-fixing
bacteria are low, generally less than 5 kg N ha-1
y-1