Background
In our Editorial of 10 Dec 2003, we challenged the claim of one of the authors of the Hungate et al. (2003) study that "we should not count on carbon storage by land ecosystems to make a massive contribution to slowing climate change," which contention is based upon the additional claim - expressed in the same Carnegie Institution press release - 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."  The recent paper of George et al. (2003) provides additional evidence that this hypothesis is fatally flawed.

The authors of the new report begin by noting that "several studies have documented a decrease in the specific rate of fine-root respiration for trees grown in elevated atmospheric CO2 (Callaway et al., 1994; BassiriRad et al., 1997; Crookshanks et al., 1998)."  Then, citing the work of Cotrufo et al. (1998) as a basis for the idea that "growth under elevated CO2 causes a decrease in the nitrogen concentration of roots," which in turn suggests "a reduction in protein concentration," they hypothesize that "the energy required for protein turnover may decline in elevated CO2 causing a reduction in maintenance respiration," and that the resultant "decrease in maintenance respiration with elevated CO2 may contribute to increases in growth respiration."

What was done
George et al. tested this hypothesis by measuring the maintenance respiration of non-growing fine roots in the absence of nutrients, while they quantified growth respiration from calculated construction costs and the observed production rates of fine roots in two major forest FACE studies: the Duke Forest study of a loblolly pine (Pinus taeda L.) plantation and the Oak Ridge National Laboratory (ORNL) Forest study of a sweetgum (Liquidambar styraciflua L.) plantation.

What was learned
Based upon unpublished data of R.J. Norby and D.W. Johnson from the ORNL Forest and data of Finzi et al. (2002) from the Duke Forest, George et al. report that - contrary to their initial assumption, as well as that of Hungate et al. - they "were unable to detect an effect of elevated CO2 on the nitrogen concentration of fine roots for either species."  Nevertheless, there was a significant reduction of fine-root maintenance respiration in the loblolly pine plantation, although there was no significant difference in this parameter between the two sweetgum CO2 treatments.  In addition, assessments of CO2-induced increases in fine-root production in the study of Matamala and Schlesinger (2000) for the loblolly pines and in the study of Norby et al. (2002) for the sweetgum trees revealed fine-root biomass increases of 87% in the first case and 77% in the second case in response to an approximate 200-ppm increase in atmospheric CO2 concentration.

What it means
In light of these observations, George et al. report that "the C:N ratio of fine roots grown in elevated CO2 was not altered and consequently did not explain the trend of reduced annual fine-root maintenance respiration and the increase in annual fine-root growth respiration for loblolly pine."  Instead, they conclude that "for these forests it appears that an increase in fine-root production is the primary factor contributing to the increase in annual growth respiration under elevated CO2."  Hence, they end up rejecting their original hypothesis, which fails primarily because of the Duke Forest and ONRL trees' refusal to comply with the faulty idea that growth under elevated CO2 must necessarily cause a decrease in the nitrogen concentrations of their roots, which is essentially the same faulty idea espoused by Hungate et al., which thus also stands rejected yet again (see our Editorial of 10 Dec 2003 for other demonstrations of its invalidity).

References
BassiriRad, H., Griffin, K.L., Reynolds, J.F. and Strain, B.R.  1997.  Changes in root NH⁺₄ and NO^-₃ absorption rates of loblolly and ponderosa pine in response to CO2 enrichment.  Plant and Soil 190: 1-9.

Callaway, R.M., DeLucia, E.H., Thomas, E.M. and Schlesinger, W.H.  1994.  Compensatory responses of CO2 exchange and biomass allocation and their effects on the relative growth rate of ponderosa pine in different CO2 and temperature regimes.  Oecologia 98: 159-166.

Cotrufo, M.F., Ineson, P. and Scott, A.  1998.  Elevated CO2 reduces the nitrogen concentration of plant tissues.  Global Change Biology 4: 43-54.

Crookshanks, M., Taylor, G. and Broadmeadow, M.  1998.  Elevated CO2 and tree root growth: contrasting responses in Fraxinus excelsior, Quercus petraea and Pinus sylvestris.  New Phytologist 138: 241-250.

Finzi, A.C., DeLucia, E.H., Hamilton, J.G., Richter, D.D. and Schlesinger, W.H.  2002.  The Nitrogen Budget of a pine forest under free air CO2 enrichment.  Oecologia 132: 567-578.

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

Matamala, R. and Schlesinger, W.H.  2000.  Effects of elevated atmospheric CO2 on fine root production and activity in an intact temperate forest ecosystem.  Global Change Biology 6: 967-979.

Norby, R.J., Hanson, P.J., O'Neill, E.G., Tschaplinski, T.J., Weltzin, J.F., Hansen, R.A., Cheng, W., Wullschleger, S.D., Gunderson, C.A., Edwards, N.T. and Johnson, D.W.  2002.  Net primary productivity of a CO2-enriched deciduous forest and the implications for carbon storage.  Ecological Applications 12: 1261-1266.