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The Long-Term Response of Forests to Atmospheric CO2 Enrichment
Volume 11, Number 28: 9 July 2008

The authors of an important new paper (Pritchard et al., 2008) note that "theory and models" have long predicted that for the majority of forest ecosystems, "insufficient soil nutrient availability will eventually constrain a sustained positive response to supra-ambient CO2 and therefore place a limit on carbon sequestration potential," which concept has come to be known as the progressive nitrogen limitation or PNL hypothesis. However, they state that data from long-term free-air-CO2-enrichment or FACE experiments "have yet to provide convincing evidence in support of the PNL hypothesis." In fact, they report that exposure to elevated concentrations of atmospheric CO2 has actually increased net primary productivity or NPP by 59%, 24%, 23% and 30% at the Rhinelander, Wisconsin (USA), Oak Ridge National Laboratory (USA), Tuscania (Italy), and Duke, North Carolina (USA) FACE sites, respectively, "with little evidence to indicate a diminished response through time," citing the analysis of Finzi et al. (2007) in this regard. But how is this possible?

One idea that was favored by many was a CO2-induced increase in nitrogen use efficiency; but "contrary to expectations" -- as with the PNL hypothesis -- the analysis of Finzi et al. revealed, in the words of Pritchard et al., that "sustained increases in NPP at the Rhinelander, Duke, and Oak Ridge National Laboratory sites was accompanied by accelerated nitrogen uptake from the soil, not by increased nitrogen use efficiency as was widely predicted." So what powers this phenomenon?

The leading hypothesis has been that atmospheric CO2 enrichment leads to greater fine-root production and increased allocation of carbon to ectomycorrhizal or ECM fungi that live in symbiotic association with the plant roots, which dual phenomenon leads to (1) the exploration of a greater volume of soil by plants in search of much needed nitrogen, as well as (2) a more thorough search of each unit volume of soil. Consequently, Pritchard et al. focused their attention on the role played by ECM fungi over a period of five years in the Duke Forest FACE study of loblolly pines based on minirhizotron observations of fungal dynamics. And what did they learn?

Summed across all years of the study, the five researchers found that the extra 200 ppm of CO2 enjoyed by the trees in the high-CO2 treatment did not influence mycorrhizal production in the top 15 cm of the forest soil, but that it increased mycorrhizal root-tip production by a whopping 194% throughout the 15-30 cm depth interval. In addition, they report that production of soil rhizomorph length was 27% greater in CO2-enriched plots than it was in the ambient-air plots.

In discussing their findings, Pritchard et al. say the CO2-induced "stimulation of carbon flow into soil has increased the intensity of root and fungal foraging for nutrients," and that "the shift in distribution of mycorrhizal fungi to deeper soils may enable perennial plant systems to acquire additional soil nitrogen to balance the increased availability of ecosystem carbohydrates in CO2-enriched atmospheres," which additional acquisition of nitrogen (N) in the CO2-enriched plots of the Duke Forest study has been determined to be approximately 12 g N per m2 per year.

In further commenting on the results of their work, Pritchard et al. write that "the notion that CO2 enrichment expands the volume of soil effectively explored by roots and fungi, and that foraging in a given volume of soil also seems to intensify, provides compelling evidence to indicate that CO2 enrichment has the potential to stimulate productivity (and carbon sequestration) in N-limited ecosystems more than previously expected." On the other hand, they also say "it is unlikely that ecosystem productivity will be stimulated by CO2 enrichment indefinitely." Be that as it may, nature has so far proved such negative hunches wrong nearly every step of the way, as scientists have probed ever deeper into this particular subject; and the five researchers thus rightly acknowledge that "only by prolonging ongoing ecosystem scale experiments will we know for sure." We whole-heartedly agree with them on this point. The long-term studies to which they refer are much too valuable to terminate before they have yielded the "final answer" to this most important question.

Sherwood, Keith and Craig Idso

References
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.

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.