Publication Date:
2012-10-10
Description:
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 C 4 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 C 4 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 μl 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.
Print ISSN:
1752-993X
Electronic ISSN:
1752-9921
Topics:
Biology
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