Background
As noted in our Editorial of 10 Dec 2003, Hungate et al. (2003) claim - by way of a Carnegie Institution press release hyping their paper - that in a future world of higher atmospheric CO2 concentration, "the availability of nitrogen [N], in forms usable by plants, will probably be too low for large increases in carbon [C] storage," ... or as van Groenigen et al. (2003) restate their contention, "reduced soil N availability under elevated CO2 may limit the plant's capacity to increase photosynthesis and thus the potential for increased soil C input."

What was done
In what serves as a significant real-world test of this claim, van Groenigen et al. say they "studied the effects of Trifolium repens (an N2-fixing legume [white clover]) and Lolium perenne [L.] [perennial ryegrass] on soil N and C sequestration in response to 9 years of elevated CO2 under FACE conditions."  Their work was conducted at the FACE facility of the Swiss Federal Institute of Technology in Eschikon 20 km northeast of Zurich, where control FACE plots were maintained at the CO2 concentration of the ambient air (~350 ppm) and CO2-enriched plots were maintained at a concentration of 600 ppm.  In addition, each FACE plot was split into a low- and high-N soil fertility treatment by applying either 140 kg N ha^-1 or 560 kg N ha^-1, respectively, to the soil following periodic cuttings of the swards.

What was learned
The scientists report that as observed in other studies (Hu et al., 2001; Kimball et al., 2002), they too "found a trend of increased total soil C under elevated CO2."  However, they also report that "although N2 fixation was a major source of N for T. repens, the presence of N2 fixation per se did not lead to higher soil N and C content compared with a low-N-fertilized L. perenne system."  Hence, they concluded that "factors other than N2 fixation exert a higher control on soil C and N stabilization in the T. repens system."

What it means
The findings of this study do not support the claim of Hungate et al., in that these real-world observations demonstrate that extra nitrogen provided in the form of either fertilizer-applied N or biologically-fixed N did not further enhance the CO2-induced sequestration of carbon in the soil of this experiment.

References
Hu, S., Chapin III, F.S., Firestone, M.K., Field, C.B. and Chiariello, N.R.  2001.  Nitrogen limitation of microbial decomposition in a grassland under elevated CO2.  Nature 409: 188-191.

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

Kimball, B.A., Kobayashi, K. and Bindi, M.  2002.  Responses of agricultural crops to free-air CO2 enrichment.  Advances in Agronomy 77: 293-368.