Letters to Nature

Nature 417, 279-282 (16 May 2002) | doi:10.1038/417279a; Received 19 July 2001; Accepted 14 February 2002

Nonlinear grassland responses to past and future atmospheric CO2

Richard A. Gill1,2, H. Wayne Polley3, Hyrum B. Johnson3, Laurel J. Anderson2,4, Hafiz Maherali1 & Robert B. Jackson1,5

  1. Department of Biology, Duke University, Durham, NC 27708-0340 North Carolina, USA
  2. Nicholas School of the Environment and Earth Sciences, Duke University, Durham, NC 27708-0340 North Carolina, USA
  3. USDA-ARS Grassland, Soil and Water Research Laboratory, Temple, Texas 76502-9601, USA
  4. Department of Botany, University of Texas, Austin, Texas 78713, USA
  5. Present addresses: Program in Environmental Science and Regional Planning, Washington State University, Pullman, Washington 99164, USA (R.A.G.); Ohio Wesleyan University, Department of Botany-Microbiology, Delaware, Ohio 43015, USA (L.J.A.)

Correspondence to: Richard A. Gill1,2 Correspondence and requests for materials should be addressed to R.A.G. (e-mail: Email: rgill@wsu.edu).

Carbon sequestration in soil organic matter may moderate increases in atmospheric CO2 concentrations (C a) as C a increases to more than 500 micromol mol-1 this century from interglacial levels of less than 200 micromol mol-1 (refs 1–6). However, such carbon storage depends on feedbacks between plant responses to C a and nutrient availability7, 8. Here we present evidence that soil carbon storage and nitrogen cycling in a grassland ecosystem are much more responsive to increases in past C a than to those forecast for the coming century. Along a continuous gradient of 200 to 550 micromol mol-1 (refs 9, 10), increased C a promoted higher photosynthetic rates and altered plant tissue chemistry. Soil carbon was lost at subambient C a, but was unchanged at elevated C a where losses of old soil carbon offset increases in new carbon. Along the experimental gradient in C a there was a nonlinear, threefold decrease in nitrogen availability. The differences in sensitivity of carbon storage to historical and future C a and increased nutrient limitation suggest that the passive sequestration of carbon in soils may have been important historically, but the ability of soils to continue as sinks is limited.