Root responses along a subambient to elevated CO2 gradient in a C3–C4 grassland

Atmospheric CO2 (Ca) concentration has increased significantly during the 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 not well understood. We measured net community root biomass over 3 years using ingrowth cores in a natural C3–C4 grassland exposed to a gradient of Ca from preglacial to future levels (230–550 μmol mol−1). Root windows and minirhizotron tubes were installed below naturally occurring 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 2 years. Community root biomass increased significantly (P<0.05) with Ca over initial conditions, with linear or curvilinear responses 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 individual 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.