What was done
Aeschlimann et al. summarize the major findings of the decade-long Swiss FACE study of monocultures of perennial ryegrass (Lolium perenne cv. Bastion) and white clover (Trifolium repens cv. Milkanova), where the air's CO2 concentration was enriched to 240 ppm above ambient during all daylight hours of the growing season in each year.

What was learned
In the words of the authors, "net assimilation at elevated CO2 in the field increased [by as much as 32%, depending on the intensity of intercepted light] not only in the short term but remained considerably stimulated for 9 nears, showing that even over a long period photosynthetic acclimation to elevated CO2 may be of minor importance," citing also the work of Ainsworth et al. (2003), who "found at the same experimental site no evidence for a decrease in the stimulation of photosynthesis in L. perenne leaves across 10 years of CO2 enrichment."  In addition, they report that this "remarkable" stimulation of net assimilation occurred even when soil nitrogen supply was low.  However, it was found that the percentage increase in harvestable biomass was much less [28% at high nitrogen and non-significant at low nitrogen], due to the propensity of the crop to overly increase root biomass, root-to-shoot ratio and the mass of pseudostems below plant cutting height.

It was additionally determined that "elevated CO2 increased net ecosystem respiration by up to 39% in both species at both low and high nitrogen supply," most probably as a result of "higher plant biomass at elevated CO2."  As a result, Aeschlimann et al. report that "elevated CO2 had only a marginal effect on the net ecosystem carbon input [to the soil], because the increased net CO2 uptake during daytime at elevated CO2 was mostly compensated for by a higher night-time respiration."  Consequently, they say that "no significant changes in soil organic carbon were found after our FACE experiment had been running for 6 and 8 years."  They did, however, find "a tendency to higher net carbon input at elevated CO2 in some years," but they note that "this is most probably not a consistent trend."  Either a longer period of observation or more numerous soil carbon measurements would have been needed to resolve this issue.

What it means
Although there was essentially no acclimation or down regulation of photosynthesis in this long-term study, no significant increase in soil carbon content was detected over the experiment's duration.  However, as demonstrated in our Editorial of 5 Mar 2003, it is sometimes necessary to conduct studies of biological responses to atmospheric CO2 enrichment for considerably longer than a decade in order to determine their true ultimate impact.  Indeed, the study of Schneider et al. (2004) on the identical L. perenne plots studied here suggests the very same thing with respect to the slowly increasing (though non-significant) trend in harvestable biomass observed in the low soil nitrogen treatment.

References
Ainsworth, E.A., Davey, P.A., Hymus, G.J., Osborne, C.P., Rogers, A., Blum, H., Nosberger, J. and Long, S.P.  2003.  Is stimulation of leaf photosynthesis by elevated carbon dioxide concentration maintained in the long term?  A test with Lolium perenne grown for 10 years at two nitrogen fertilization levels under Free Air CO2 Enrichment (FACE).  Plant, Cell and Environment 26: 705-714.

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.