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

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 m(2)) open top chambers to investigate effects of twice ambient CO2 concentration (720 muL L-1) 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 water potential (Psi(leaf)) was enhanced 24 - 30% under elevated CO2 in the major warm- and cool-season grass species of the SGS (Bouteloua gracilis, C-4, 28.5%; Pascopyrum smithii, C-3, 24.7%; Stipa comata, C-3, 30.4%), and the degree of responsiveness in Psi(leaf) to elevated CO2 did not differ between C-3 and C-4 plant functional types, but did differ between C-3 species. Water-use efficiency (WUE; g aboveground biomass harvested/kg water consumed) was 43% higher in elevated (6.10) than ambient (4.27) CO2 plots over the study period. Results suggest that a future, elevated CO2 environmentmay 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, increases in deep soil water may eventually favor deeper-rooted over shallow-rooted species.