Excess nitrogen deposition: issues for consideration.

This paper briefly reviews some major mechanisms by which deposition of inorganic N compounds from the atmosphere could be damaging forest and natural ecosystems. Twelve issues which needed further discussion or research were identified. These were: has N deposition increased; what is a N-saturated ecosystem; can the time of onset of N saturation be predicted; can fertiliser experiments simulate the effects of atmospheric deposition; are there relationships between N input and N leaching; is N deposition leading to acidification; does high N input lead to toxicity symptoms in trees; does N input increase tree susceptibility to stress; does N input induce nutrient deficiency; does increasing N affect natural plant communities; what are the effects on aquatic ecosystems; can a 'critical load' for protection of ecosystems be defined? There is a brief critical discussion of each issue. It is concluded that there is not enough understanding of ecosystem function to define a critical load objectively, but that limits can be defined for some ecosystems.