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.