Enhanced plant biomass accumulation in response to elevated atmospheric CO₂ concentration could dampen the future rate of increase in CO₂ levels and associated climate warming. However, it is unknown whether CO₂-induced stimulation of plant growth and biomass accumulation will be sustained or whether limited nitrogen (N) availability constrains greater plant growth in a CO₂-enriched world^{1, 2, 3, 4, 5, 6, 7, 8, 9}. Here we show, after a six-year field study of perennial grassland species grown under ambient and elevated levels of CO₂ and N, that low availability of N progressively suppresses the positive response of plant biomass to elevated CO₂. Initially, the stimulation of total plant biomass by elevated CO₂ was no greater at enriched than at ambient N supply. After four to six years, however, elevated CO₂ stimulated plant biomass much less under ambient than enriched N supply. This response was consistent with the temporally divergent effects of elevated CO₂ on soil and plant N dynamics at differing levels of N supply. Our results indicate that variability in availability of soil N and deposition of atmospheric N are both likely to influence the response of plant biomass accumulation to elevated atmospheric CO₂. Given that limitations to productivity resulting from the insufficient availability of N are widespread in both unmanaged and managed vegetation^{5, 7, 8, 9}, soil N supply is probably an important constraint on global terrestrial responses to elevated CO₂.

1. Department of Forest Resources,
2. Department of Ecology, Evolution and Behavior, University of Minnesota, St Paul, Minnesota 55108, USA
3. Department of Biology, University of Wisconsin–Eau Claire, Eau Claire, Wisconsin 54701, USA
4. School of Natural Resources and Environment, University of Michigan, Ann Arbor, Michigan 48109, USA
5. School of Biological Sciences, University of Nebraska, Lincoln, Nebraska 68588, USA
6. Department of Ecology, Evolution, and Environmental Biology, Columbia University, New York, New York 10027, USA

Correspondence to: Peter B. Reich¹ Correspondence and requests for materials should be addressed to P.B.R. (Email: preich@umn.edu).

Received 10 October 2005 | Accepted 29 November 2005

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