Document: JUL-3-34-71

Biological N-fixation in California annual grassland: Responses to elevated CO2 and effects on community productivity.

VERVILLE, J.H.* ^1,2 and C.B.FIELD ³

Stanford University, Stanford, CA, USA ¹
University of Utah, Salt Lake City, UT, USA ²
Carnegie Institution of Washington, Stanford, CA, USA ³

Abstract:
The capacity of many terrestrial ecosystems to increase carbon uptake and storage under elevated atmospheric CO₂ is influenced by the availability of limiting resources, particularly nitrogen (N). Biological N-fixation is a major source of N to many terrestrial systems, and the response of N-fixing organisms to elevated CO₂ is likely to affect N-availability. I measured the growth and N-fixation responses of two annual grassland legumes to elevated CO₂, and measured the effects of legume presence on community productivity and nutrient pools under ambient and elevated CO₂, and across range of N-addition levels. Elevated CO₂ increased the growth of the legume Lotus wrangelianus, but did not significantly stimulate growth of the legume Vicia sativa, which was more abundant than L. wrangelianus in the communities. Nitrogen addition caused a decline in the biomass of both legume species. Elevated CO₂ did not significantly affect total legume biomass or the amount of N fixed. Communities with legumes had 28-55% more N in aboveground plant tissue than communities without legumes, with the greatest difference at low N-addition. However, N pools in legume tissue more than accounted for the differences in total community N, suggesting that legume presence did not increase N availability to non-fixers. Total aboveground productivity was comparable in communities grown with and without legumes, and there was no difference between communities in how productivity responded to elevated CO₂ or N addition.

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This abstract is being presented at: 2:15 PM in session:
Oral Session #30: Effects of Elevated Carbon Dioxide.