eLearning

can be repeated precisely, quality assurance is dif-
ficult to achieve unless archived soils are available
from earlier sampling periods to provide confidence
in repeatability among periods. We note that all
three approaches to examining changes in forest N
contents examine net changes, and these net
changes result from possible difference in both in-
put and output rates. Some studies of each type
have sampled only a small portion of the total N
pool in forests (such as 0–10-cm-depth mineral
soil), and accretion estimates from such studies may
reflect shifts among pools within the forest rather
than novel inputs.
PAIRED PLOT STUDIES
Son and Gower (1992) measured N in vegetation
and soil to a depth of 30 cm into the mineral soil in
a 28-year-old common garden plantation in Wis-
consin, USA. Red oak (Quercus rubra L.) plots had
the lowest N content (approximately 3420 kg
N/ha), and Norway spruce (Picea abies (L.) Karst.)
had the highest (4540 kg N/ha). The annual rate of
divergence ranged from 10 to 40 kg N ha-1
y-1
among species; differences of approximately 20 kg
Nha-1
y-1
or greater were significant at P , 0.05.
The divergence could represent occult N inputs, but
differences in N pools in lower soil horizons, as well
as N losses, would need to be characterized for a
complete picture. We suggest this study warrants
moderate confidence as a measure of differences in
N inputs.
Fisher and Eastburn (1974) examined two
former prairie soils that were planted with four
species of trees after years of agricultural farming.
When the trees were 20 years old, the apparent N
accretion under red pine (Pinus resinosa) averaged
107 kg N ha-1
y-1
(P , 0.02 based on two replicate
plots, each sampled in three subplots with compos-
ited samples from 10 soil cores). The comparable
estimate for Japanese larch (Larix leptolepis Gord.)
was71kgNha-1
y-1
(P 5 0.06). No accretion of N
was evident under either tulip tree (Liriodendron
tulipifera L.) or basswood (Tilia americana L.). Any
change in the N content of the adjacent fields dur-
ing the 20 years would alter the N accretion esti-
mate. The fields were managed with a rotation of
corn, corn, and legume hay, and we expect that
fertilizer inputs probably were similar to N losses for
little net change. Some of the occult N accretion
could have resulted from applications of anhydrous
ammonia to surrounding fields (R.F. Fisher, per-
sonal communication), but this would not explain
the discrepancy in N accretion between the conifers
and hardwoods. Another possibility is that the