Elevated CO2increases soil moisture and enhances plant water relations in a long-term field study in semi-arid shortgrass steppe of Colorado

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 concentration (720 μL 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 (Ψleaf) 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 Ψleaf to elevated CO2 did not differ between C3 and C4 plant functional types, but did differ between C3 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 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, increases in deep soil water may eventually favor deeper-rooted over shallow-rooted species.