BassiriRad 9728785 Nitrogen (N) and phosphorus (P) are nutrients that most often limit plant production in natural and agricultural ecosystems. Limited availability of these nutrients is also a major factor influencing long-term plant and ecosystem responses to rising atmospheric CO2 levels i.e., the commonly observed short-term increase in plant biomass may not be sustained over the long-term unless availability and/or acquisition capacity for nutrients increases in concert with carbon (C) gain. Therefore, it is critical to obtain a mechanistic understanding of whether elevated CO2 can elicit compensatory adjustments that would circumvent nutrient limitation of plant growth. Compensatory adjustment such as (a) increased mycorrhizal development, (b) increased root N and P absorption capacity, and (c) root growth can enable plants to meet increased nutrient demand under high CO2 yet the extent of these adjustments in response to CO2 enrichment is unknown. In addressing how nutrient availability could interact with plants and ecosystem responses to high CO2 several questions must be considered: Will plant demand for N and P increase in response to CO2 enrichment ? (Demand as a concept requires careful definition on temporal and spatial scales which will be addressed here.) If nutrient demand does increase under elevated C02, what compensatory adjustment(s) will plants make to overcome this greater demand? Will N and P uptake scale properly with measured changes in relative growth rate (RGR) and photosynthetic N and P utility? Does growth nutrient status affect the magnitude of the compensatory adjustments in response to CO2? Do these compensatory mechanisms scale with CO2-induced changes in stages of development? Do plants of various functional groups e.g., C3 vs. C4, woody vs. herbaceous vs. grasses, or arbuscular mycorrhizae vs. ectomycorrhizae differ in their nutrient uptake responses to elevated CO2? Does a better plant capacity to scale up nutrient uptake at elevated CO2 confer enhanced competitive ability? The proposed research will address these questions with an emphasis on root and mycorrhizal N and P uptake kinetics. The PI will use both field and laboratory approaches to test our hypotheses under the most realistic conditions possible. The field work will be conducted at six currently funded national FACE sites providing a comprehensive cross comparison of different ecosystems and functional groups. At each field site, N and P uptake capacity will be measured on intact roots of mature plants subjected to factorial combination of two CO2 treatments in the presence or absence of mycorrhizas. The field experiments will provide a unique opportunity to compare the response of these compensatory adjustments to elevated CO2 levels among different ecosystems, life forms, and functional groups of plants in a competitive setting. Greenhouse-laboratory experiments using species similar to those from the field will examine the more detailed questions related to adjustments in uptake kinetics, relative changes in root vs. mycorrhizal uptake kinetics, and plant physiological demand. The data will be integrated into a comprehensive functional balance model which will incorporate physiological and biomass allocation parameters.
