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Title: Root Responses Along a Subambient to Elevated CO2 Gradient in a C3-C4 Grassland

Author
item ANDERSON, LAUREL - OHIO WESLEYAN UNIVERSITY
item Derner, Justin
item Polley, Herbert
item GORDON, WENDY - TEXAS PARKS & WILDLIFE
item EISSENSTAT, DAVID - PENN STATE UNIVERSITY
item JACKSON, ROBERT - DUKE UNIVERSITY

Submitted to: Global Change Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/1/2009
Publication Date: 12/17/2009
Citation: Anderson, L.J., Derner, J.D., Polley, H.W., Gordon, W.S., Eissenstat, D.M., Jackson, R.B. 2009. Root Responses Along a Subambient to Elevated CO2 Gradient in a C3-C4 Grassland. Global Change Biology. 16:454-468.

Interpretive Summary: One of the most clear global changes is the increase in concentration of atmospheric carbon dioxide (CO2). The effects of this increase on aboveground responses of grassland plants has received much research attention, but belowground responses have been little studied. We evaluated root biomass and production, respiration and lifespan of a dominant warm-season grass to a gradient in atmospheric CO2 spanning pre-Industrial to predicted end-of-the 21st century levels. Root biomass increased as atmospheric CO2 increased, but root production decreased as well as lifespan. These results emphasize that aboveground and belowground responses can be quite different to increases in atmospheric CO2.

Technical Abstract: Atmospheric CO2 (Ca) concentration has increased significantly during last 20,000 years, and is projected to double this century. Despite the importance of belowground processes in the global carbon cycle, community-level and single species root responses to rising Ca are poorly understood. We measured community root biomass over three growing seasons using ingrowth cores in a natural C3-C4 grassland exposed to a gradient of Ca from pre-glacial to future levels (230 to 550 µmol mol-1). Root windows and minirhizotron tubes were installed below stands of the C4 perennial grass Bothriochloa ischaemum and its roots were measured for respiration, carbohydrate concentration, specific root length (SRL), production, and lifespan over two years. Community root biomass increased significantly (P < 0.05) with Ca over initial conditions, and the shape of the response was linear or curvilinear depending on sample date. In contrast, B. ischaemum produced significantly more roots at subambient than elevated Ca in minirhizotrons. The lifespan of roots with five or more neighboring roots in minirhizotron windows decreased significantly at high Ca, suggesting that after dense root growth depletes soil resource patches, plants with carbon surpluses readily shed these roots. Root respiration in B. ischaemum showed a curvilinear response to Ca under moist conditions in June 2000, with the lowest rates at Ca < 300 µmol mol-1 and peak activity at 450 µmol mol-1 in a quadratic model. B. ischaemum roots at subambient Ca had higher SRLs and slightly higher carbohydrate concentrations than those at higher Ca, which may be related to drier soils at low Ca. Our data emphasize that belowground responses of plant communities to Ca can be quite different from those of the component species, and suggest that complex interactions between and among roots and their immediate soil environment influence the responses of root physiology and lifespan to changing Ca.