Aims Rising atmospheric CO2 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 CO2 enrichment for ANPP will vary with soil type and may be reduced if water limitation is low. Little is known about CO2 effects on ANPP of Panicum virgatum, a perennial C4 tallgrass and potential bioenergy crop. We hypothesized that if water limitation is minimized, (i) CO2 enrichmentwould not increase P. virgatum ANPP because photosynthetic rates of this C4 grass would not increase and because decreased transpiration at elevated CO2 would provide little additional benefit in increased soil moisture and (ii) soil type will have little effect on P. virgatum CO2 responses because of high overall soil moisture.
Methods Growth and leaf physiology of P. virgatum cv. \u27Alamo\u27 were studied as plants established for 4 years on silty clay and clay soils along a 250 to 500 J.l1 1-1 gradient in atmospheric C02 located in central Texas, USA. Plants were watered to replace evapotranspiration, fertilized with NO3NH4 and P2O5 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 CO2 except in the planting year. However, biomass removed with clipping strongly increased with CO2 in years 2 and 3, suggesting that CO2 enrichment increased the early- to mid-season growth of establishing P. virgatum but not later regrowth or that of fully established plants. Furthermore, CO2 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 CO2 enrichment. CO2 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 CO2 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 CO2 enrichment when fully established but interacted with clipping to alter allocation patterns during establishment. Soil type effects on ANPP-C02 responses will likely become more apparent when water is more limiting
