ROOT BIOMASS OF INDIVIDUAL SPECIES, AND ROOT SIZE CHARACTERISTICS AFTER FIVE YEARS OF CO2 ENRICHMENT ON NATIVE SHORTGRASS STEPPE

Authors: Lecain, Daniel; Morgan, Jack; MILCHUNAS, DANIEL - COLORADO STATE UNIVERSITY; Mosier, Arvin; Nelson, Jim; Smith, David

Submitted to: Plant and Soil
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/20/2005
Publication Date: 2/15/2006
Citation: Lecain, D.R., Morgan, J.A., Milchunas, D.G., Mosier, A.R., Nelson, J.A., Smith, D.P. 2006. Root biomass of individual species, and root size characteristics after five years of CO2 enrichment on native shortgrass steppe. Plant and Soil Journal. 279:219-228.

Interpretive Summary: Atmospheric CO2 concentration has steadily increased over the past 100 years or so, primarily due to combustion of fossil fuels. There is great concern that increases in greenhouse gases, like CO2, will cause severe and non-reversible changes in the earth’s climate and Carbon (C) cycling. Grasslands occupy approximately 40% of the terrestrial biomes on earth, and are expected to have some of the largest ecological changes to global change. Scientists from USDA-ARS, Colorado State University and the University of Colorado, Boulder, conducted a 5 year study on the Colorado prairie (shortgrass steppe). Open-top chambers which enclosed 16 m2 of ground area were placed over native prairie; CO2 concentration in half of the chambers was increased to 2X the ambient concentration. Many ecological and biological parameters were measured over the 5 year study; these results have been published in many scientific journals. In the shortgrass steppe, about 90% of the plant matter is under-ground. Therefore, root responses to global change are very important. At the end of the 5-year study, we conducted an in-depth assessment of the effect of elevated CO2 on root weight and root size, using large soil monoliths and root cores. Roots were separated by species for root weights, when they could be identified by attachment to crowns. A commercial “hair detangler” greatly aided in the separation of roots. Results were somewhat surprising. Despite earlier reports of as much as a 100% increase in above-ground plant matter under elevated CO2 (primarily S. comata) there were only small increases in root biomass. Only one species Stipa comata (Needle and Thread grass) had much of an increase in root mass. However, we found a significant increase in very fine roots (<0.1mm diameter) in the 0-10 cm soil depth layer. Based on earlier reports of a nearly 400% increase in S. comata seedlings in the elevated CO2 plots, we propose that the increase in fine roots is due to shallow, fine roots of S. comata seedlings. These results suggest that S. comata is becoming more dominant under elevated CO2, both above and below-ground. We conclude that increasing atmospheric CO2 will have only small effects on standing root biomass and root length and diameter of most shortgrasss steppe species. However, the potential increased competitive ability of Stipa comata, a low forage quality species, could alter the ecosystem from the current dominant, high forage quality species, Bouteloua gracilis. B. gracilis is very well adapted to the frequent droughts of the shortgrass steppe. Increased competitive ability of less desirable plant species under increasing atmospheric CO2 will have large implications for long-term sustainability of grassland ecosystem

Technical Abstract: Information from field studies investigating the responses of roots to increasing atmospheric CO2 is limited and somewhat inconsistent, due partly to the difficulty in studying root systems in situ. In this report, we present root biomass by species and root length and diameter after five years of CO2 enrichment (~720 µmol mol-1) in large (16 m2 ground area) open-top chambers placed over a native shortgrass steppe in Colorado, USA. Total root biomass in 150 L soil monoliths and root biomass of the three dominant grass species of the site were not significantly affected by elevated CO2, Despite a nearly 100% increase in root biomass of Stipa comata in the 0-20cm soil depth of elevated vs. ambient CO2 chambers, the CO2 effect was not statistically significant (P=0.14). However, there was a 37% increase in fine root length under elevated CO2 in the 0-10cm soil depth layer. Few treatment differences in root length or diameter were detected in lower, 10cm depth increments, down to 80 cm. Other reports from this study suggest that the increase in fine roots is primarily from improved seedling recruitment of Stipa comata under elevated CO2. These results reflect the root status integrated over two wet, two dry and one normal precipitation years and nearly one complete cycle of root turnover on the shortgrass steppe. We conclude that increasing atmospheric CO2 will have only small effects on standing root biomass and root length and diameter of most shortgrasss steppe species. However, the potential increased competitive ability of Stipa comata, a low forage quality species, could alter the ecosystem from the current dominant, high forage quality species, Bouteloua gracilis. B. gracilis is very well adapted to the frequent droughts of the shortgrass steppe. Increased competitive ability of less desirable plant species under increasing atmospheric CO2 will have large implications for long-term sustainability of grassland ecosystems.