What was done
The authors studied the effect of atmospheric CO2 enrichment on soil C sequestration via a meta-analysis based on the results of 41 published and unpublished studies.

What was learned
"Elevated CO2," in the words of the six scientists who conducted the analysis, "had no effect on soil C in ecosystems receiving up to 30 kg/ha/yr of N." However, they say that it "increased soil C by 2.1% per year at N additions between 30 and 150 kg/ha/yr and by 2.9% per year at N additions >150 kg/ha/yr." To provide some context for these rates of N fertilization, they say the first of the N-addition categories is "comparable to maximum atmospheric N depositions in the United States and most of the European Union," while the second is "typical of extensive agriculture in the United States," and the third is "typical for intensive agriculture in the European Union."

What it means
Van Groenigen et al. say their analysis "provides empirical corroboration for the largely untested hypothesis that large C accumulations only occur with increased inputs or reduced losses of N." Calling the PNL hypothesis "largely untested," however, is clearly incorrect. Lou et al. (2006), for example, tested it via a meta-analysis based on data obtained from considerably more scientific publications (104 vs. 41) than those employed by van Groenigen et al.; and they found significant CO2-induced increases in plant, litter and soil C contents, as well as plant, litter and soil N contents, which findings led them to conclude there may be "a long-term trend of terrestrial C sequestration in response to rising atmospheric CO2 concentrations."

This conclusion is clearly at odds with that of van Groenigen et al.; but it is tentatively supported (or, at the very least, in no way threatened) by the findings of the first six years of the impressive long-term FACE studies of the sweetgum forest described by Norby and Iverson (2006) and the loblolly pine forest described by Finzi et al. (2006), as well as by the first seven years of the equally illustrious open-top chamber study of the scrub oak forest described by (Hungate et al., 2006). The findings of these experiments suggest that even longer studies of trees growing out-of-doors and rooted in the ground are what are needed to ultimately resolve the issue. Finzi et al. (2006), for example, end their paper by stating that "only direct observations through time will definitively answer this question," although Norby and Iversen (2006) are quick to caution that "CO2 enrichment experiments in [their] sweetgum plantation and other forests may never [our italics] be of sufficient duration to resolve these long-term adjustments between the C and N cycles."

Because of this conundrum, it is important to seek out other natural phenomena that may shed light upon the PNL hypothesis; and that is precisely what Luo et al. (2006) did when they considered the impacts of primary and secondary succession on ecosystem C and N cycling. Their brief review of the subject readily reveals that as environmental and biological conditions change over time in ways that allow for greater rates of primary production and enhanced C inputs to soils, ecosystems experiencing such changes somehow find ways to bolster soil N inputs to accommodate the extra C. Hence, there is an independent and objectively-verifiable reason to believe that the environmental change of atmospheric CO2 enrichment (which enhances ecosystem C acquisition) should somehow stimulate ecosystem N acquisition to whatever extent is needed to support the sequestration of some of the extra C thus obtained within the ecosystem’s soil and plant reservoirs.

In conclusion, it would appear that the balance of evidence weighs somewhat heavily against the PNL hypothesis, and in continued favor of the greening of the earth scenario.

Reference
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.

Hungate, B.A., Johnson, D.W., Dijkstra, P., Hymus, G., Stiling, P., Megonigal, J.P., Pagel, A.L., Moan, J.L., Day, F., Li, J., Hinkle, C.R. and Drake, B.G. 2006. Nitrogen cycling during seven years of atmospheric CO2 enrichment in a scrub oak woodland. Ecology 87: 26-40.

Luo, Y., Hui, D. and Zhang, D. 2006. Elevated CO2 stimulates net accumulations of carbon and nitrogen in land ecosystems: A meta-analysis. Ecology 87: 53-63.

Norby, R.J. and Iversen, C.M. 2006. Nitrogen uptake, distribution, turnover, and efficiency of use in a CO2-enriched sweetgum forest. Ecology 87: 5-14.