Background
Over the years, certain scientists have periodically opined that the stimulation of plant growth provided by rising atmospheric CO2 concentrations will produce a negative feedback that reduces the amount of inorganic nitrogen available to plants and thereby thwarts their ability to grow ever more vigorously as the aerial fertilization effect of the ongoing rise in the air's CO2 concentration becomes ever more powerful.  See our Editorial of 10 Dec 2003 that introduces this concept and reviews a number of studies that convincingly refute it, as well as two other studies we have subsequently reviewed that do the same (Holmes et al., 2003; van Groenigen et al., 2003).

What was done
In a FACE study that increased the mean atmospheric CO2 concentration to only about 100 ppm above ambient, and based on seasonal sampling over a five-year period, the authors studied the influence of elevated CO2 on soil carbon and nitrogen (N) pools and mineralization in a fertilized (P,K,S) sheep-grazed New Zealand pasture of mixed grass, clover and forbs.

What was learned
With the minimal increase in atmospheric CO2 employed in their study, Ross et al. had difficulty detecting much change in many of the parameters they measured.  However, they say their results "suggest increased availability of N, probably because of increased inputs from N-fixing clovers."  As has been observed previously (Ross et al., 1996; van Kessel et al., 2000), for example, they report "a marked increase in the proportion of legumes [which fix nitrogen from the atmosphere and transfer it to the soil, where it can be used by other plants] in the pasture herbage under elevated CO2 during the early years of the trial," which they say "could explain subsequent increases in rates of soil net N mineralization."  Or as they re-state this conclusion in another place, "the larger pool of mineralizable N in the elevated-CO2 soil than in the ambient-CO2 soil over the last year of the trial probably resulted, as found elsewhere (Soussana and Hartwig, 1996), from a preceding increased input of symbiotically fixed N from the increased legume component."

What it means
Once again, science triumphs over speculation.

References
Holmes, W.E., Zak, D.R., Pregitzer, K.S. and King, J.S.  2003.  Soil nitrogen transformations under Populus tremuloides, Betula papyrifera and Acer saccharum following 3 years exposure to elevated CO2 and O3.  Global Change Biology 9: 1743-1750.

Soussana, J.F. and Hartwig, U.A.  1996.  The effects of elevated CO2 on symbiotic N2 fixation: a link between the carbon and nitrogen cycles in grassland ecosystems.  Plant and Soil 187: 321-332.

van Groenigen, K.-J., Six, J., Harris, D., Blum, H. and van Kesssel, C.  2003.  Soil ¹³C-¹⁵N dynamics in an N2-fixing clover system under long-term exposure to elevated atmospheric CO2.  Global Change Biology 9: 1751-1762.