Background
Over the years, certain scientists and climate alarmists have claimed that the stimulation of plant growth provided by the ongoing rise in the atmosphere's CO2 concentration will produce a negative feedback that reduces the amount of inorganic nitrogen available to plants, thereby thwarting their ability to grow ever more vigorously as time progresses. Hungate et al. (2003), for example, contend that when atmospheric CO2 enrichment increases the soil carbon/nitrogen (C/N) ratio, "decomposing microorganisms require more nitrogen." They further state that this latter effect "can reduce nitrogen mineralization," which they say is "the main source of nitrogen for plants." Hence, they conclude, as described in the press release that accompanied the publication of their paper, that in a high-CO2 world of the future "the availability of nitrogen, in forms usable by plants, will probably be too low for large increases in carbon storage," which they claim will ultimately lead to more rapid global warming.

What was done
In a study whose findings are germane to this hypothesis, Graaff et al. "determined the long-term (9 years) impact of elevated CO2 on N mineralization of Lolium perenne and Trifolium repens plant material grown at ambient and elevated CO2" in the Swiss FACE study that was established at Eschikon in 1993.

What was learned
The researchers report that "elevated CO2 did not change plant N mineralization in any of the soils," including both "the fertilized and leguminous systems" and "the low soil N availability systems." With respect to the high soil N systems, in fact, Schneider et al. (2004) report that "in 1993, the CO2 response of harvested biomass was 7.2%, increasing to 32% in 2002," which also led to large increases in plant demand for nitrogen over this period (Hartwig et al., 2002). In addition, even the low soil N systems exhibited an increasing (though non-significant) trend in biomass production, suggesting that if the experiment had continued even longer, this trend too would likely have gained statistical significance.

What it means
In light of these several real-world observations, plus others archived in our Subject Index under the heading Nitrogen (Cycling), it seems clear that atmospheric CO2 enrichment will not lead to reductions in nitrogen mineralization in the world's soils. In fact, the results of the study of Jastrow et al. (2005), as well as the results of the many other studies they review, indicate that enriching the air with CO2 actually tends to increase soil nitrogen concentrations.

References
Hartwig, U.A., Luscher, A., Nosberger, J. and van Kessel, C. 2002. Nitrogen-15 budget in model ecosystems of white clover and perennial ryegrass exposed for four years to elevated atmospheric pCO2. Global Change Biology 8: 194-202.

Hungate, B.A., Dukes, J.S., Shaw, M.R., Luo, Y. and Field, C.B. 2003. Nitrogen and climate change. Science 302: 1512-1513.

Jastrow, J.D., Miller, R.M., Matamala, R., Norby, R.J., Boutton, T.W., Rice, C.W. and Owensby, C.E. 2005. Elevated atmospheric carbon dioxide increases soil carbon. Global Change Biology 11: 2057-2064.

Schneider, M.K., Luscher, A., Richter, M., Aeschlimann, U., Hartwig, U.A., Blum, H., Frossard, E. and Nosberger, J. 2004. Ten years of free-air CO2 enrichment altered the mobilization of N from soil in Lolium perenne L. swards. Global Change Biology 10: 1377-1388.