1. Department of Ecology, Evolution and Behavior, 100 Ecology, 1987 Upper Buford Circle, University of Minnesota, St. Paul, Minnesota 55108, USA
2. These authors contributed equally to this work.

Correspondence to: Christopher M. Clark^1,2 Correspondence and requests for materials should be addressed to C.C. (Email: clark134@umn.edu).

Rates of atmospheric deposition of biologically active nitrogen (N) are two to seven times the pre-industrial rates in many developed nations because of combustion of fossil fuels and agricultural fertilization^{1, 2}. They are expected to increase similarly over the next 50 years in industrializing nations of Asia and South America². Although the environmental impacts of high rates of nitrogen addition have been well studied^{3, 4, 5, 6, 7, 8}, this is not so for the lower, chronic rates that characterize much of the globe. Here we present results of the first multi-decadal experiment to examine the impacts of chronic, experimental nitrogen addition as low as 10 kg N ha^-1 yr^-1 above ambient atmospheric nitrogen deposition (6 kg N ha^-1 yr^-1 at our site). This total input rate is comparable to terrestrial nitrogen deposition in many industrialized nations². We found that this chronic low-level nitrogen addition rate reduced plant species numbers by 17% relative to controls receiving ambient N deposition. Moreover, species numbers were reduced more per unit of added nitrogen at lower addition rates, suggesting that chronic but low-level nitrogen deposition may have a greater impact on diversity than previously thought. A second experiment showed that a decade after cessation of nitrogen addition, relative plant species number, although not species abundances, had recovered, demonstrating that some effects of nitrogen addition are reversible.

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