Human activities, related primarily to agricultural practices and the combustion of fossil fuels for energy and transport, have caused steep increases in global emissions of reactive nitrogen (Nr) over the last 50 years. Atmospheric nitrogen (N) deposition derived from these sources represents a major threat to natural ecosystems around the world, leading to changes in structure, function and the associated biodiversity.

Although substantial progress has been made in past decades, especially in Europe and North America, major uncertainties remain regarding the global perspective. In the last 30 years, Europe and North America have made great progress in assessing the problem, although the knowledge base is greater for impacts on flora than fauna; in many other parts of the world, however, the required datasets and assessments in many cases do not yet exist.

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