Roots of CO₂-Enriched Trees Seek Out Needed Nitrogen

Iversen et al. (2011) write that "elevated CO₂ has increased the NPP [net primary production] of tree stands in four large-scale forested free-air CO₂ enrichment (FACE) experiments (Norby et al., 2005)," and they say that it has also led to "increased uptake of mineral N [nitrogen] from the soil in three of the four forested FACE experiments, even those that were initially N-limited (Finzi et al., 2007)." Thus, they set about to determine how the CO₂-enriched trees may have accomplished this impressive feat.

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)."

Additional References
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.