As a result of increasing anthropogenic nitrogen deposition, N
availability in many forest ecosystems, which are
normally N-limited, has been enhanced. We discuss the impacts of this
increased N availability on the
ectomycorrhizal (ECM) symbiosis which is generally regarded as an
adaptation to nutrient limited conditions.
Nitrogen deposition can influence fruit-body formation by ECM fungi, the
production and distribution of the
extraradical mycelium in the soil and the formation of ectomycorrhizas.
Available data from long-term N deposition studies indicate that the
most prominent effects might be
discernible above-ground (i.e. on the formation of fruit bodies).
‘Generalist’ species, forming a symbiosis with a
wide range of tree species, seem to be less affected by increased N availability
than ‘specialist’ species, especially
those living in symbiosis with conifers. However, the importance of
below-ground investigations to determine the
impacts of N deposition on the ECM symbiosis must not be underestimated.
Culture experiments show an
optimum N concentration for the formation of extraradical mycelium
and mycorrhizas. Often, negative effects
only become visible at comparatively high N concentrations, but the
use of a few easily cultivated species of ECM
fungi, which are adapted to higher N concentrations, undermines our ability
So far, N deposition experiments in the field have only shown minor
changes in the below-ground mycorrhizal
population, as estimated from the investigation of mycorrhizal root tips.
However, effects on the ECM mycelium,
which is the main fungal component in terms of nutrient uptake, cannot
excluded and need further consideration.
Because the photoassimilate supply from the plant to the fungal partner
is crucial for the maintenance of the
ECM symbiosis, we discuss the possible physiological implications of
increasing N inputs on the allocation of C
to the fungus. Together with ultrastructural changes, physiological effects
might precede obvious visible changes
and might therefore be useful early indicators of negative impacts of
increasing N inputs on the ECM symbiosis.