Submitted to: Plant and Soil
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 5, 2001
Publication Date: N/A
Interpretive Summary: Shoots and leaves of warm-season plants grow better with increased concentrations of atmospheric carbon dioxide (CO2) when soils are dry. However, it is unclear if root growth exhibits the same pattern. Soil water had greater effects than did CO2 on root growth of the two dominant warm-season grasses from tallgrass prairie, little bluestem and big bluestem. However, root growth responses to CO2 was similar whether soil water was limiting or abundant. Greatest increases in root growth occurred in very fine roots. This pattern of root growth may influence nutrient cycling in soil, because fine roots die and decompose quickly and take up more nutrients and water from soil than large roots.
Aboveground growth of C4 plants responds more strongly to atmospheric CO2 concentration when soil water is limiting than abundant. Whether the same is true of root growth and morphology, however, has received little attention. We investigated interactive effects of CO2 and soil water availability on root growth and morphology of C4 grasses. Seedlings of the two dominant C4 grasses from tallgrass prairie were grown for 8 weeks in an environmental chamber with CO2 concentrations of 368 (ambient) and 203 (subambient) umol mol-1. Seedlings were maintained at either high (ca. 90%) or low (ca. 50%) soil relative water content (RWC). Interactions between CO2 and RWC affected leaf length and shoot and total mass with proportionally greater responses to increases in CO2 at the high than low RWC treatment. In contrast, RWC had substantially greater effects than CO2 on root variables (45-101% vs. 29-36%). The responsiveness of these C4 grass root systems to CO2 was similar whether soil water was limiting or abundant. The relative distribution of root surface area, number of root tips and length and volume of roots were significantly affected by CO2 with greatest proportional increases (39-58%) occurring in very fine (0-0.3 mm) roots. Increased fine root growth with increases in CO2 concentrations may significantly influence 1) nutrient cycling, as these roots may have greater turnover rates, and 2) nutrient and water uptake from soil. Therefore, the human-induced increases in CO2 may have increased soil exploitation by C4 grasses irrespective of soil water availability and potentially increased competitiveness for soil resources.