Drawing primarily on studies in Europe, the authors find the main mechanisms of N deposition on terrestrial biodiversity are: direct foliar impacts; eutrophication; acidification; negative effects of reduced N; and increased susceptibility to secondary stress and disturbance factors such as drought, frost, pathogens or herbivores.

The relation of several of these mechanisms to aquatic ecosystems is also described, as is the relative lack of N impact studies on faunal species/communities compared to floral ones.

In addition, the factors that moderate N impacts on ecosystems are considered. They are categorized as: (1) the duration and total amount of the N inputs; (2) the chemical and physical form of the airborne N input; (3) the intrinsic sensitivity to the changes in N availability of the plant and animal species present; (4) the abiotic conditions (such as the ability of soils and waters to neutralize acidification effects); and (5) the past and present land use or management.

The increased susceptibility of plants (or animal) species to stresses and disturbances, induced by enhanced atmospheric N loads, is highly dependent of the large differences in the physiological functioning of individual species. Therefore, the generalization of the effects of N deposition over a range of ecosystems is hardly, if at all, possible, although these impacts have been demonstrated to be of major importance in some ecosystems.

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