COS 42-10 - Elevated [CO2] and warming differentially impact carbon isotope discrimination and tissue N concentration in five grassland species

Tuesday, August 4, 2009: 4:40 PM
Sendero Blrm III, Hyatt
Emma L. Sage1, David G. Williams2, Elise Pendall3, Daniel R. LeCain4, Jack A. Morgan4 and Ron Follett5, (1)Program in Ecology and Department of Botany, University of Wyoming, Laramie, WY, (2)Department of Botany, University of Wyoming, Laramie, WY, (3)Botany, University of Wyoming, Laramie, WY, (4)Rangeland Resources Research Unit, USDA-ARS, Fort Collins, CO, (5)USDA-ARS, Ft. Collins, CO
Background/Question/Methods: We measured carbon isotope discrimination (D13C) and tissue N content in above ground biomass over four years at the Prairie Heating and CO2 Enrichment (PHACE) experiment in southeastern Wyoming, USA. This factorial experiment combined FACE (ambient and elevated [600 ppm] CO2 concentration), experimental warming (1.5˚C daytime, 3˚C nighttime) and irrigation to evaluate direct, indirect and interactive effects of global change factors on semi-arid grassland dynamics and production. We sampled species representing three functional types: a C4 grass Bouteloua gracilis; a C3 taprooted forb Sphaeralcea coccinea; and three C3 graminoids Pascopyrum smithii, Carex eleocharis, and Hesperostipa comata.

Results/Conclusions: We found D13C was reduced similarly by about 2‰ for all species under elevated compared to ambient [CO2], suggesting that at least for C3 species stomatal closure under elevated [CO2] reduced CO2 supply relative to carboxylation capacity. The ratio of leaf internal to ambient CO2 concentration (Ci/Ca) calculated from D13C values in P. smithii, the dominant C3 graminoid, was negatively correlated with tissue N content in elevated [CO2], but not in ambient [CO2].  Tissue N content on a per unit mass basis was lower in the elevated (1.4±0.07%) compared to the ambient (1.8±0.07%) [CO2] treatment. Experimental warming by itself did not alter D13C for any species, but warming lessened effects of elevated [CO2] on this integrated gas exchange trait. Irrigation had little effect on D13C or tissue N content. The apparent interaction between elevated [CO2] and warming on D13C and tissue N content was unexpected and is potentially attributed to 1) accelerated phenological development with warming; 2) warming-induced changes in leaf structure and anatomy; or 3) indirect and interactive effects of warming on soil nitrogen supply.

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