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 fertilization1,2. 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 studied3,4,5,6,7,8, 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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We thank C. Stevens, J. Galloway, S. Collins, K. Suding, J. Hille Ris Lambers, and J. Fargione for their comments on the manuscript, and T. Mielke and the summer interns at Cedar Creek for their work. This project was supported by the Long-Term Ecological Research Network funded by the National Science Foundation.
Author Contributions D.T. envisioned, set up and managed the long-term experiment. C.C. performed the analysis and wrote much of the paper. Both authors discussed the results, and developed and commented on the manuscript.
This file provides additional detail on the statistical analyses and results that support the main text, and includes Supplementary Notes, Supplementary Tables 1-3 and Supplementary Figure 1 with Legends. (PDF 172 kb)
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Clark, C., Tilman, D. Loss of plant species after chronic low-level nitrogen deposition to prairie grasslands. Nature 451, 712–715 (2008). https://doi.org/10.1038/nature06503
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