Roots of CO2-Enriched Trees Seek Out Needed Nitrogen
Iversen, C.M., Hooker, T.D., Classen, A.T. and Norby, R.J. 2011. Net mineralization of N at deeper soil depths as a potential mechanism for sustained forest production under elevated [CO2]. Global Change Biology 17: 1130-1139.
Working in the CO2-enriched sweetgum (Liquidambar styraciflua L.) plantation located in Oak Ridge, Tennessee (USA), Iversen et al. used 15N isotope pool dilution methodology to measure potential gross N cycling rates in laboratory incubations of soil from four depth increments to 60 cm in order to determine whether N is available for root acquisition in deeper soil and to see if it may have altered N cycling rates.
When all was said and done, the four researchers concluded that "deeper rooting distributions in response to CO2 enrichment were likely associated with relatively greater N availability in deeper soil rather than altered N cycling rates under elevated CO2," providing evidence that "N mineralization at depth in the soil, combined with increased root exploration of the soil volume under elevated CO2, may be more important than changes in potential gross N cycling rates in sustaining forest responses to rising atmospheric CO2."
In considering these results, when atmospheric CO2 enrichment provides an opportunity for trees to enhance their growth rates, it also seems to provide a way for them to find the extra nitrogen they need to do so. And, in the words of Iversen et al., "in addition to internal ecosystem N recycling at depth, external sources of N may also lead to greater root proliferation in deep soil," as they note that "root access to inorganic N found in groundwater at >2 m depth in CO2-enriched chambers may have sustained tree production under elevated CO2 in a scrub-oak system in Florida, USA (McKinley et al., 2009)."
Finzi, A.C., Norby, R.J., Calfapietra, C., Gallet-Budynek, A., Gielen, B., Holmes, W.E., Hoosbeek, M.R., Iversen, C.M., Jackson, R.B., Kubiske, M.E., Ledford, J., Liberloo, M., Oren, R., Polle, A., Pritchard, S., Zak, D.R., Schlesinger, W.H. and Ceulemans, R. 2007. Increases in nitrogen uptake rather than nitrogen-use efficiency support higher rates of temperate forest productivity under elevated CO2. Proceedings of the National Academy of Sciences, USA 104: 14,014-14,019.
McKinley, D.C., Romero, J.C., Hungate, B.A., Drake, B.G. and Megonigal, J.P. 2009. Does deep soil N availability sustain long-term ecosystem responses to elevated CO2? Global Change Biology 15: 2035-2048.
Norby, R.J., DeLucia, E.H., Gielen, B., Calfapietra, C., Giardina, C.P., King, S.J., Ledford, J., McCarthy, H.R., Moore, D.J.P., Ceulemans, R., De Angelis, P., Finzi, A.C., Karnosky, D.F., Kubiske, M.E., Lukac, M., Pregitzer, K.S., Scarasci-Mugnozza, G.E., Schlesinger, W.H. and Oren, R. 2005. Forest response to elevated CO2 is conserved across a broad range of productivity. Proceedings of the National Academy of Sciences 102: 18,052-18,056.