High loads of nitrogen to spruce and beech forests can result in
a complete inhibition of NO3− uptake by the
roots
of the trees. This conclusion is based on (a) a comparison of
a
field site continuously exposed to high loads of N
and a N-limited site, (b) the results of N fertilization of a
N-limited field site, and (c) laboratory experiments under
controlled environmental conditions. From fertilization experiments in
the field it appears that NH4+ uptake might
become inhibited subsequent to an excessive uptake of NH4+.
Apparently, the inhibition of NO3− uptake by
high
loads of N to forests is a consequence of an accumulation of organic amino
compounds in the roots originating from
phloem transport from the shoot to the roots. These amino compounds seem
to signal the N demand of the shoot
to the roots. At present this function cannot be attributed to an
individual organic amino compound in beech or
spruce, but Gln is a likely candidate in both species among other
compounds, e.g. Glu in spruce or Asp in beech
trees. Direct inhibition of NO3− uptake
by NH4+ can be excluded from the present studies.
The mechanism(s) by
which elevated levels of particular organic amino compounds
interact with NO3− uptake remains to be elucidated.
This (these) mechanism(s) seem to affect NO3−
influx rather than NO3− efflux. As a consequence
of this (these)
mechanism(s), spruce and beech trees can prevent, within a certain
physiological window, N over-nutrition when
the roots are exposed to excessive amounts of inorganic N. However,
inhibition of NO3− and NH4+
uptake by the
roots makes more N available for leaching into the ground water
and, in addition, for soil microbial processes that
result in the production and re-emission of volatile N compounds into the
atmosphere.
At the ‘Höglwald’ site, continuously exposed to high
loads of N, >20% of the N input from throughfall into
the spruce and beech plots is re-emitted as NO and N2O. However,
the NO to N2O ratio is highly dependent on
the tree species, with a preference for NO in the spruce and a preference
for N2O in the beech plot. Since at least
part of the NO emitted from the soil will be converted inside the
canopy in the presence of ozone to NO2 that might
then be absorbed by the leaves, the portion of the N in the throughfall
that will be released from the forest by
gaseous N emission is higher in the beech than in the spruce plot.
Leaching of NO3− into the ground water is high
in the spruce, but minute in the beech plot. However, this positive
effect of beech on ground water quality is
achieved at the expense of an enhanced release of radiatively active N
gases into the troposphere.