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Effect of Atmospheric CO2 Enrichment on Nitrogen Fixation by Legumes
Hungate, B.A., Stiling, P.D., Dijkstra, P., Johnson, D.W., Ketterer, M.E., Hymus, G.J., Hinkle, C.R. and Drake, B.G. 2004. CO2 elicits long-term decline in nitrogen fixation. Science 304: 1291.

Atmospheric CO2 enrichment generally enhances the nitrogen-fixing abilities of most legumes; for several examples, see Nitrogen Fixation (Herbaceous Plants and Woody Plants in our Subject Index. As with all general rules, however, there are sometimes exceptions to the norm; and we here report on one that has been observed within this specific category of plant response to atmospheric CO2 enrichment.

What was done
In an auxiliary experiment conducted within the open-top-chambers designed to study the response of a native scrub-oak community in central coastal Florida (USA) to a 350-ppm increase in the air's CO2 concentration, Hungate et al. measured nitrogen fixation by the leguminous vine Galactia elliotti Nutt. over a period of seven years.

What was learned
Over the course of their experiment, the researchers found that the extra CO2 "initially increased but later suppressed N fixation, contrary to the expected response." As for the reason for this anomalous behavior, they state that "reduced availability of the micro-nutrient molybdenum [Mo], a key constituent of nitrogenase, best explains this reduction in N fixation." Specifically, they note that "Mo deficiency in N-fixing plants has been documented, particularly in sandy acidic soils similar to the scrub-oak soil," and that "elevated CO2 increased nutrient accumulation in oak biomass and in organic forms in the soil, potentially reducing their availability to G. elliotti." What is more, they report that elevated CO2 did indeed substantially decrease foliar concentrations of Mo in G. elliotti.

What it means
A climate alarmist might suggest that these observations "caution against expecting increased biological N fixation to fuel terrestrial carbon accumulation," as Hungate et al. put it. However, it is most interesting to note that, in the very same issue of Science in which Hungate et al.'s paper was published, Palmer et al. (2004) report that "fossil fuel combustion and fertilizer production have doubled the global rate of nitrogen fixation ... fertilizing remote portions of the planet," and they say that "because our planet will be over-populated for the foreseeable future and natural resource consumption shows no signs of slowing," this human modification of the environment "will only increase."

Consequently, even if there may be isolated instances of CO2-induced decreases in nitrogen fixation by certain plants growing on certain soils, humanity is more than compensating for this phenomenon on a worldwide basis, in terms of supplying the extra nitrogen that will be needed to keep earth's plants sequestering ever more carbon in their tissues and the soils upon which they grow as the air's CO2 content continues to rise.

Palmer, M., Bernhardt, E., Chornesky, E., Collins, S., Dobson, A., Duke, C., Gold, B., Jacobson, R., Kingsland, S., Kranz, R., Mappin, M., Martinez, M.L., Micheli, F., Morse, J., Pace, M., Pascual, M., Palumbi, S., Reichman, O.J., Simons, A., Townsend, A. and Turner, M. 2004. Ecology for a crowded planet. Science 304: 1251-1252. Reviewed 8 March 2006