Author(s): Benjamin D. Duval 1,2,*, Paul Dijkstra 1,2, Bert G. Drake 3, Dale W. Johnson 4, Michael E. Ketterer 5, J. Patrick Megonigal 3, Bruce A. Hungate 1,2
Many studies have evaluated the effects of elevated CO2 on nitrogen cycling, and focused on the hypothesis that tree growth response to elevated CO2 may be limited by N availability, or change with N use efficiency -, but the impact of CO2 on elements other than N has been studied less frequently. The nutrients P, K and Ca can also limit plant productivity , , non-N nutrients can limit N2 fixation and C storage , and changes in Al, Mn and Fe concentrations might affect the availability of other mineral nutrients in soils . To fully assess the impact of elevated CO2 on ecosystem nutrient cycling, it is important to evaluate effects on all elements that are necessary for plant nutrition and elements that control the availability of other nutrients in the soil system.
Photosynthesis and growth are often stimulated by elevated CO2 in C3 plants , , often leading to more biomass production. Increased growth increases nutrient demands . It has been suggested that nutrients become more limiting for growth over time and can limit terrestrial C uptake . Increased production of carbohydrates in plants is suggested to reduce element concentrations in plants . Elevated CO2 generally reduces plant N concentration, but increased growth does not inherently dilute the concentration of other elements in plant tissues -. In sweet gum (Liquidambar styraciflua ), Johnson et al.  found significant declines in foliar Fe concentration with elevated CO2 . At the POP-EUROFACE CO2 experiment, there was no change in poplar leaf K or Ca concentrations, while Mg concentration actually increased in those trees . A cross-experiment evaluation of elevated CO2 by Natali et al.  showed significantly lower Fe concentration in sweet gum at the Duke FACE site, decreased Al, V and Fe concentrations in sweet gum at the Oak Ridge FACE site, but increased Mn and Mo concentrations in Quercus myrtifolia at the Smithsonian Institution Elevated CO2 site in Florida. Lastly, a recent meta-analysis of 14 tree species and 10 nutrient elements found that elevated CO2 lowers Cu, Fe, K, Mg, P and S concentrations, but only at high N availability .
Element availability varies by soil type and ecosystem. Soil element availability is a function of soil organic matter content (SOM) and pH, with elements generally less adsorbed to metal oxides and SOM in acid soils . Elevated CO2 has been shown to increase P availability, possibly a function of decline in SOM in the experimental plots . Elevated CO2 has also been implicated in reducing leaching of soil N and P from upper soil layers . A recent study found that trace metal concentrations increased in soils exposed to elevated CO2 at Duke FACE and Oak Ridge FACE, but decreased under elevated CO2 for...