Human activity has more than doubled the rate of creation of reactive nitrogen on the land surface of the earth over natural levels, with much of this change occurring in recent decades. Major drivers behind this increase are population growth, agricultural intensification, and emissions of nitrogen pollutants to the atmosphere from the combustion of fossil fuels. As a result, nitrogen is accumulating in the landscape and in water at unprecedented levels, and is connected to a host of environmental problems including effects on natural ecosystems and on human health. For example, eutrophication caused by excessive inputs of N from landscapes to the oceans is one of the greatest factors altering water quality in coastal ecosystems worldwide.

This synthesis is the final report from the International SCOPE Project on Nitrogen Transport and Transformations: A Regional and Global Analysis. SCOPE (the Scientific Committee on Problems of the Environment) authorized the Nitrogen Project because of the need to better understand how humans have altered nitrogen cycling globally and at the scale of large regions. The project has synthesized information through a series of workshops over the past 8 years, involving over 250 scientists from over 20 different nations. Papers in this volume explore the extent to which human activity has affected the nitrogen cycle in terrestrial regions and in the world's oceans, and discuss the implications of accelerated nitrogen cycling for nature and society.