eLearning

Dan Binkley,
Yowhan Son,
and David W. Valentine

ABSTRACT
The nitrogen (N) cycle of forest ecosystems is un-
derstood relatively well, and few scientists expect
that major revisions will be necessary; most current
work on N cycling focuses on improving the preci-
sion estimates of pools and fluxes, or measuring the
magnitudes of well-known pools in response to
management or disturbances. However, in the past
few decades more than a dozen articles in refereed
journals have claimed very high rates of N input, far
beyond the rates expected for known sources of N.
In this review, we summarize the literature on N
accretion rates in forests that lack substantial con-
tributions from symbiotic N-fixing plants. We cri-
tique each study for the strength of the experimen-
tal design behind the estimate of N accretion and
consider whether unexpectedly large inputs of N
really occur in forests. Only 6 of 24 estimates of N
accretion had strong experimental designs, and
only 2 of these 6 yielded estimates of .5kg
Nha-1
y-1
. The high accretion estimates with a
strong experimental design come from repeated
sampling at the Walker Branch watersheds in Ten-
nessee, where N accretion rates ranged from 50 to
80 kg N ha-1
y-1
over 15 years after harvesting. At
the same location, an unharvested stand showed no
significant change. We conclude that there is no
widespread evidence of high rates of occult N input
in forests. Too few studies have carefully tested for
balanced N budgets in forests (inputs minus outputs
plus change in storage), and we recommend that at
least a few of these studies be undertaken on soils
that permit high precision sampling.