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Title: Elevated CO2 increases soil moisture and enhances plant water relations in a long-term field study in the semi-arid shortgrass steppe of Northern Colorado

Author
item NELSON, J - COLORADO STATE UNIVERSITY
item Morgan, Jack
item Lecain, Daniel
item Mosier, Arvin
item MILCHUNAS, D - COLORADO STATE UNIVERSITY
item PARTON, W - COLORADO STATE UNIVERSITY

Submitted to: Plant and Soil
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/26/2003
Publication Date: 2/1/2004
Citation: Nelson, J.A., Morgan, J.A., Lecain, D.R., Mosier, A.R., Milchunas, D.G., Parton, W.A. 2004. Elevated co2 increases soil moisture and enhances plant water relations in a long-term field study in the semi-arid shortgrass steppe of colorado. Plant And Soil Journal. 259:169-179.

Interpretive Summary: Rising concentrations of atmospheric carbon dioxide have prompted the initiation of numerous studies designed to evaluate how higher carbon dioxide will affect important world ecosystems. Only a handful of long-term field studies have yet investigated the impact of rising carbon dioxide on unperturbed native ecosystems, and fewer yet have evaluated how CO2 effects plant ecosystems indirectly through its impacts on soil and plant water relations. This study, conducted on a shortgrass prairie on the western edge of the Great Plains in northern Colorado, demonstrates that while CO2 has the capacity to affect plant directly by stimulating photosynthesis, it also appears improve plant water relations. The results from this study suggest that this secondary effect of CO2 may be more important than the direct photosynthetic responses of grasslands to CO2, and may enhance productivity in water-limited grasslands worldwide.

Technical Abstract: Increasing atmospheric [CO2] has potentially significant impacts on the dynamics of water use and conservation in semi-arid rangelands. In this study we used large (15.5 m2) open top chambers to investigate effects of twice ambient [CO2] on plant and soil water relations of semi-arid shortgrass steppe (SGS) of northeastern Colorado from 1997 to 2001. Seasonal average soil moisture throughout the soil profile (0-15, 15-45, 45-75, 75-105 cm) was increased under elevated CO2 compared to ambient CO2 for much of the study period. When averaged across years, the greatest relative increase (elevated vs. ambient) in soil moisture occurred in the 75-105 cm depth increment (16.4%). Averaged over the study period, leaf xylem pressure potential (Yleaf) was enhanced 24 ' 30% under elevated CO2 in the major warm- and cool-season grass species of the SGS (Bouteloua gracilis, C4, 28.5%; Pascopyrum smithii, C3, 24.7%; Stipa comata, C3, 30.4%), and the degree of responsiveness in Yleaf to elevated CO2 did not differ between C3 and C4 plant functional types, but did differ between C3 species. Water-use efficiency (WUE; g phytomass produced/ kg water consumed) was 43% higher on average in elevated (6.10) than ambient (4.27) CO2 plots over the study period. Results suggest that a future, elevated CO2 environment may result not only in increased plant productivity due to improved WUE, but also lead to increased water drainage and deep soil moisture storage in this semi-arid grassland ecosystem. This, along with the ability of the major grass species to maintain a favorable water status under elevated CO2, should result in the SGS being less susceptible to prolonged periods of drought. However, species compositional changes may occur with deeper-rooted species being favored over shallow-rooted species.