Productivity of well-watered Panicum virgatum does not increase with CO_(2) enrichment

Aims Rising atmospheric CO_(2) has been shown to increase aboveground net primary productivity(ANPP)in water-limited perennial grasslands,in part by reducing stomatal conductance and transpiration,thereby reducing depletion of soil moisture.However,the benefits of CO_(2) enrichment for ANPP will vary with soil type and may be reduced if water limitation is low.Little is known about CO_(2) effects on ANPP of Panicum virgatum,a perennial C4 tallgrass and potential bioenergy crop.We hypothesized that if water limitation is minimized,(i)CO_(2) enrichment would not increase P.virgatum ANPP because photosynthetic rates of this C4 grass would not increase and because decreased transpiration at elevated CO_(2) would provide little additional benefit in increased soil moisture and(ii)soil type will have little effect on P.virgatum CO_(2) responses because of high overall soil moisture.Methods Growth and leaf physiology of P.virgatum cv.‘Alamo’were studied as plants established for 4 years on silty clay and clay soils along a 250 to 500 ll l-1 gradient in atmospheric CO_(2) located in central Texas,USA.Plants were watered to replace evapotranspiration,fertilized with NO_(3)NH_(4) and P_(2)O_(5) and clipped to standard height during mid-season.Important Findings ANPP increased through the third year of growth.Soil moisture(0–20 cm),ANPP,tiller numbers and leaf area index were 8–18%higher on the clay than on the silty clay soil.ANPP did not increase with CO_(2) except in the planting year.However,biomass removed with clipping strongly increased with CO_(2) in years 2 and 3,suggesting that CO_(2) enrichment increased the early-to mid-season growth of establishing P.virgatum but not later regrowth or that of fully established plants.Furthermore,CO_(2) enrichment differentially affected two components of ANPP in years 2 and 3,increasing tiller mass and reducing tiller numbers.This reallocation of resources in clipped P.virgatum suggested increased meristem limitation of productivity with CO_(2) enrichment.CO_(2) enrichment had little effect on photosynthesis but increasingly reduced stomatal conductance and transpiration as the plants established.As a result,water use efficiency became increasingly coupled to CO_(2) as leaf area increased during establishment.These results suggest that for well-watered and clipped P.virgatum,ANPP differed between soil types,was not affected by CO_(2) enrichment when fully established but interacted with clipping to alter allocation patterns during establishment.Soil type effects on ANPP-CO_(2) responses will likely become more apparent when water is more limiting.