9801217 Vitousek Empirical and theoretical work on the feedbacks between the terrestrial biosphere and rising levels of atmospheric carbon dioxide (CO2) has demonstrated that the capacity of many ecosystems to increase carbon (C) uptake and storage is strongly influenced by the availability of limiting resources, particularly nitrogen (N). Biological N-fixation is a major source of N to many terrestrial systems and is responsive to changes in environmental conditions. As a result, the response of N fixers to elevated CO2 is likely to affect ecosystem responses through changes in N-availability. Two experiments will be carried out in a California annual grassland ecosystem. The first will measure the timing and magnitude of growth and N-fixation under elevated CO2 to determine whether growth responses can be equated with changes in biological N-inputs. Biomass, allocation, and N-fixing activity of 3 legumes grown under ambient and elevated CO2 will be measured over the course of the growing season. Experiment 2 will test the effects of elevated CO2 on N-fixing and non-fixing plants grown together and separately along a gradient of N-availability. Primary measurements include biomass and allocation responses, the contribution of plant N derived from N-fixation, and changes in tissue chemistry of fixers and non-fixers.
