Finzi and Schlesinger (2003) measured and analyzed pool sizes and fluxes of inorganic and organic nitrogen in the forest floor and top 30 cm of mineral soil during the first five years of differential atmospheric CO2 treatment of a stand of initially 13-year-old loblolly pine trees at the Duke Forest FACE facility in the Piedmont region of North Carolina (USA), where half of the experimental plots are maintained at a mean CO2 concentration that is 200 ppm above ambient. In doing so, they found that the extra CO2 significantly increased the input of carbon (C) and nitrogen (N) to the forest floor, as well as to the mineral soil in which the trees were growing. However, they report "there was no statistically significant change in the cycling rate of N derived from soil organic matter under elevated CO2" and that "neither the rate of net N mineralization nor gross ¹⁵NH4+ dynamics were significantly altered by elevated CO2." In fact, they could find "no statistically significant difference in the concentration or net flux of organic and inorganic N in the forest floor and top 30-cm of mineral soil after 5 years of CO2 fumigation," adding that "microbial biomass was not a larger sink for N." Based on these findings, they actually rejected their own original hypothesis, which was essentially the same as the progressive nitrogen limitation hypothesis, i.e., that the extra CO2 provided to the experimental plots would significantly increase the rate of N immobilization by the soil microbial communities found within the CO2-enriched FACE arrays and thereby lead to a reduction in the magnitude of the growth stimulation that was initially manifest in the CO2-enriched treatment.

Working at the same location, Schafer et al. (2003) measured net ecosystem exchange (NEE) and net ecosystem production (NEP) during the third and fourth years of the long-term CO2 enrichment study being conducted there. They found that the extra 200 ppm of CO2 supplied to the loblolly pine trees within the CO2-enriched FACE arrays increased the entire canopy's net uptake of CO2 (NEE) by fully 41%, and that canopy NEP was increased by 44%. In addition, they determined that 87% of the extra NEP "was sequestered in a moderately long-term C pool in wood." This large increase in solidly-sequestered carbon is truly amazing, especially in light of the declaration of Finzi and Schlesinger (2003) that the soil at the Duke Forest FACE site is in "a state of acute nutrient deficiency that can only be reversed with fertilization," which, of course, is not provided.

Three years later, Finzi et al. (2006) tested the progressive nitrogen limitation (PNL) concept "using data on the pools and fluxes of C and N in tree biomass, microbes and soils," which were obtained from the first six years of the Duke Forest FACE study. As was the case three years earlier, it was once again found that "there was no reduction in the average stimulation of net primary production by elevated CO2," even though "significantly more N was immobilized in tree biomass and in the O [soil] horizon under elevated CO2." Also, and "in contrast to the PNL hypothesis," as they describe it, "microbial-N immobilization did not increase under elevated CO2, and although the rate of net N mineralization declined through time, the decline was not significantly more rapid under elevated CO2." In addition, the twelve researchers report that "mass balance calculations demonstrated a large accrual of ecosystem N capital," and they state that the rate of extra N accrual was "much greater than the estimated rate of N input via atmospheric deposition or heterotrophic N fixation," noting further that "there are no plant species capable of symbiotic N fixation in this ecosystem." In other words, by some unknown means the loblolly pine trees obtained the extra N they needed to stave off the negative effects predicted by the PNL hypothesis, possibly, in the words of Finzi et al., by roots "actively taking up N and redistributing N from deeper in the soil profile."

After another two years had passed, Moore et al. (2006) reported finding "a sustained increase in basal area increment over the first 8 years of the experiment," which varied between 13 and 27% with variations in weather and the timing of growth. What is more, they say "there was no evidence of a decline in the relative enhancement of tree growth by elevated CO2 as might be expected if soil nutrients were becoming progressively more limiting," which would normally be expected, considering the unfertilized state of the soil in which the experiment was being conducted.

Two years later, Pritchard et al. (2008a) published the results they had obtained from minirhizotrons they employed to characterize the influence of the extra 200 ppm of CO2 on the fine roots of the Duke Forest loblolly pines over the six-year period 1998-2004. Averaged over all six years, they found that it had increased average fine-root standing crop by 23%, which compares well with the overall stimulation of tree net primary productivity of 18-24% observed over the period 1996-2002. Consequently, in light of their noting that "the positive effects of CO2 enrichment on fine root growth persisted 6 years following minirhizotron tube installation (8 years following initiation of the CO2 fumigation)," as they describe it, there was once again no hint of any progressive nitrogen limitation to the stimulatory effect of atmospheric CO2 enrichment in a situation where one would have expected to have encountered it. In partial explanation of this important positive finding, Pritchard et al. concluded their report by stating that the distal tips of fine roots are "the primary site for initiation of mycorrhizal partnerships which are critical for resource acquisition and could also influence whether or not forests can sustain higher productivity in a CO2-enriched world."

In a study designed to further explore this aspect of the ongoing long-term Duke Forest FACE experiment, Pritchard et al. (2008b) focused their attention on the role played by ectomycorrhizal (ECM) fungi over a period of five years, based on minirhizotron observations of fungal dynamics. 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 the 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 in the CO2-enriched plots of the Duke Forest FACE 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 Pritchard et al. on this latter 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.

References
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. and Schlesinger, W.H. 2003. Soil-nitrogen cycling in a pine forest exposed to 5 years of elevated carbon dioxide. Ecosystems 6: 444-456.

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

Luo, Y., Su, B., Currie, W.S., Dukes, J.S., Finzi, A., Hartwig, U., Hungate, B., McMurtrie, R.E., Oren, R., Parton, W.J., Pataki, D.E., Shaw, M.R., Zak, D.R. and Field, C.B. 2004. Progressive nitrogen limitation of ecosystem responses to rising atmospheric carbon dioxide. BioScience 54: 731-739.

Moore, D.J.P., Aref, S., Ho, R.M., Pippen, J.S., Hamilton, J.G. and De Lucia, E.H. 2006. Annual basal area increment and growth duration of Pinus taeda in response to eight years of free-air carbon dioxide enrichment. Global Change Biology 12: 1367-1377.

Pritchard, S.G., Strand, A.E., McCormack, M.L., Davis, M.A., Finzi, A.C., Jackson, R.B., Matamala, R., Rogers, H.H. and Oren, R. 2008a. Fine root dynamics in a loblolly pine forest are influenced by free-air-CO2-enrichment: a six-year-minirhizotron study. Global Change Biology 14: 588-602.

Pritchard, S.G., Strand, A.E., McCormack, M.L., Davis, M.A. and Oren, R. 2008b. Mycorrhizal and rhizomorph dynamics in a loblolly pine forest during 5 years of free-air-CO2-enrichment. Global Change Biology 14: 1-13.

Schafer, K.V.R., Oren, R., Ellsworth, D.S., Lai, C.-T., Herrick, J.D., Finzi, A.C., Richter, D.D. and Katul, G.G. 2003. Exposure to an enriched CO2 atmosphere alters carbon assimilation and allocation in a pine forest ecosystem. Global Change Biology 9: 1378-1400.