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Ten Years of Free-Air CO2 Enrichment of Perennial Ryegrass
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.

What was done
Swards of perennial ryegrass (Lolium perenne L. cv. Bastion) were established in monoculture in 1992 at the Swiss FACE facility (Hebeisen et al., 1997) at Eschikon near Zurich and maintained under atmospheric CO2 concentrations of either 360 or 600 ppm for a full decade.  Half of the swards were yearly supplied with low amounts of nitrogen fertilizer (10 g N m-2 year-1 in 1993 and 14 g N m-2 year-1 from 1994 to 2002), while half of them were yearly supplied with high amounts (40 g N m-2 year-1 in 1993 and 56 g N m-2 year-1 from 1994 to 2002).  In addition, small sections of the swards received 15N-enriched NH4NO3 (ammonium and nitrate equally enriched, diluted in H2O) at each fertilization.

What was learned
In the words of the authors, "in 1993, the CO2 response of harvested biomass was 7.2%, increasing to 32% in 2002."  At low N, however, they report that the CO2 response "varied annually."  Nevertheless, it too exhibited a slowly increasing (though non-significant) trend, suggesting that given enough time, it might have gained statistical significance as well.  Additionally, Schneider et al. report that "at high N supply, more N was mobilized from the soil after long-term exposure to elevated CO2 than after ambient CO2," in contrast to the suggestion of Hungate et al. (2003) that just the opposite would likely occur.  At low N, however, the Swiss team writes that "the reduced availability of N constantly limited the harvestable biomass to elevated CO2 throughout the experiment," more in harmony with Hungate et al.'s suggestion.  Nevertheless, as noted above, there was a tenuous indication that this limitation may have been slightly reduced over the course of the 10-year study, and that a still longer experiment may be needed to resolve the issue in the case of low-N soils; for as we demonstrate in our Editorial of 5 Mar 2003, two or more decades may well be required to resolve some of these knotty issues.

What it means
Growth responses to long-term atmospheric CO2 enrichment in woody plants have often been found to decrease somewhat over time, as noted in our Editorial cited above, even when the plants are adequately supplied with nutrients.  In this study of perennial ryegrass, however, just the opposite occurred over a period of ten years, with no indication that the steadily increasing biomass response to elevated CO2 had yet reached its maximum.  It is thus somewhat sad to see this long-term study terminated, without knowing just how high the biomass response may have ultimately risen, or whether the biomass response of the ryegrass in the low-nitrogen treatment might have likewise been increasing, but at a much reduced rate.

To answer these important questions, which are the ones that must be answered to ascertain what will actually happen to natural and agro-ecosystems over the next century or more of continued atmospheric CO2 increase, we must have the resolve and the monetary commitment to assure that experiments such as this one continue for perhaps twice as long.  As Schneider et al. describe the situation, "the experiments reported here convincingly demonstrate that there are feedback mechanisms in the soil which are only revealed after several years of exposure to elevated CO2," which leads them to conclude that "long-term experiments are, thus, an essential prerequisite for understanding how ecosystem functioning is affected by elevated atmospheric CO2 and N supply."

Hebeisen, T., Luscher, A., Zanetti, S. et al.  1997.  Growth response of Trifolium repens L. and Lolium perenne L. as monocultures and bi-species mixture to free air CO2 enrichment and management.  Global Change Biology 3: 149-160.

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

Reviewed 6 October 2004