Volume 12, Number 18: 6 May 2009
In the introduction to their paper summarizing nine years of work at the Duke Forest Free-Air CO2 Enrichment (FACE) experiment in North Carolina (USA), where portions of an aggrading loblolly pine (Pinus taeda) plantation had been continuously exposed to an extra 200 ppm of CO2 since 1996, Lichter et al. (2008) note that progressive nitrogen limitation (PNL) may "accompany C sequestration in plants and soils stimulated by CO2 fertilization, gradually attenuating the CO2 response," after which they describe what they learned about this PNL hypothesis over the following nine years.
First of all, the nine researchers report that their data pertaining to forest-floor carbon pools indicate the existence of "a long-term steady-state sink" of about 30 g C per m2 per year, which represents, in their words, "a substantial increase in forest-floor C storage under elevated CO2 (i.e. 29%)," and which they attribute to "increased litterfall and root turnover during the first 9 years of the study." Secondly, down below the forest floor, they say that of the mineral soil C formed during the past 9 years, "approximately 20% has been allocated to stable pools that will likely remain protected from microbial activity and associated release as CO2."
A third important finding of the research team was "a significant widening of the C:N ratio of soil organic matter in the upper mineral soil under both elevated and ambient CO2," which suggests, as they describe it, that "enhanced rates of soil organic matter decomposition are increasing mineralization and uptake to provide the extra N required to support the observed increase in primary productivity under elevated CO2." At the Duke Forest FACE site, Pritchard et al. (2008) say this CO2-induced increase in productivity amounts to approximately 30% annually; and they add that there is "little evidence to indicate a diminished response through time," citing the analysis of Finzi et al. (2007), who find the same to be true at the long-term forest FACE studies being conducted at Rhinelander, Wisconsin (USA), Oak Ridge National Laboratory (USA), and Tuscania (Italy).
Contrary to the early expectations of many scientists (and most climate alarmists), it would thus appear that many of earth's forests that are thought to have access to less-than-adequate soil nitrogen supplies may indeed be able to acquire the extra nitrogen they need to maintain the sizable increases in their growth rates that are driven by elevated concentrations of atmospheric CO2. In the case of North Carolina's Duke Forest, for example, "even after nine years of experimental CO2 fertilization," as Lichter et al. describe it, "attenuation of the CO2-induced productivity enhancement has not been observed," as has also been noted to be the case by Finzi et al. (2006). And this finding at this location is extremely significant, because the growth of pine-hardwood forests in the southeastern United States often removes so much nitrogen from the soils in which they grow that they induce what Finzi and Schlesinger (2003) have described as "a state of acute nutrient deficiency that can only be reversed with fertilization," which operation, however, was not employed in the Duke Forest FACE study.
Sherwood, Keith and Craig Idso
Finzi, A.C., Moore, D.J.P., DeLucia, E.H., Lichter, J., Hofmockel, K.S., Jackson, R.B., Kim, H.-S., Matamala, R., McCarthy, H.R., Oren, R., Pippen, J.S. and Schlesinger, W.H. 2006. Progressive nitrogen limitation of ecosystem processes under elevated CO2 in a warm-temperate forest. Ecology 87: 15-25.
Finzi, A.C., Norby, R.J., Calfapietra, C., Gallet-Budynek, A., Gielen, B., Holmes, W.E., Hoosbeek, M.R., Iversen, C.M., Jackson, R.B., Kubiske, M.E., Ledford, J., Liberloo, M., Oren, R., Polle, A., Pritchard, S., Zak, D.R., Schlesinger, W.H. and Ceulemans, R. 2007. Increases in nitrogen uptake rather than nitrogen-use efficiency support higher rates of temperate forest productivity under elevated CO2. Proceedings of the National Academy of Sciences, USA 104: 14,014-14,019.
Finzi, A.C. and Schlesinger, W.H. 2003. Soil-nitrogen cycling in a pine forest exposed to 5 years of elevated carbon dioxide. Ecosystems 6: 444-456.
Lichter, J., Billings, S.A., Ziegler, S.E., Gaindh, D., Ryals, R., Finzi, A.C., Jackson, R.B., Stemmler, E.A. and Schlesinger, W.H. 2008. Soil carbon sequestration in a pine forest after 9 years of atmospheric CO2 enrichment. Global Change Biology 14: 2910-2922.
Pritchard, S.G., Strand, A.E., McCormack, M.L., Davis, M.A. and Oren, R. 2008. Mycorrhizal and rhizomorph dynamics in a loblolly pine forest during 5 years of free-air-CO2-enrichment. Global Change Biology 14: 1-13.