The Progressive Nitrogen Limitation Hypothesis: Notoriously Famous ... but Fading Fast

The progressive nitrogen limitation hypothesis posits that low concentrations of soil nitrogen will gradually curtail the ability of the productivity-enhancing effect of atmospheric CO₂ enrichment to maintain increased plant growth and ecosystem carbon sequestration rates as time progresses (Hungate et al., 2003; Luo et al., 2004). In introducing their impressive contribution to the subject, however, Hofmockel et al. (2011) report the observational fact that "several free-air CO₂ enrichment (FACE) experiments in North America have shown a continual stimulation in forest productivity under elevated CO₂ over time scales nearly reaching a decade (Finzi et al., 2006; Norby and Iversen, 2006; Zak et al., 2007; McCarthy et al., 2010)." And in their most recent examination of the effects of elevated CO₂ on nitrogen (N) cycling in the Duke Forest - where they indicate that elevated atmospheric CO₂ concentrations have "consistently stimulated forest productivity" throughout the decade-long experiment being conducted there - they go on to provide "an integrated understanding" of this phenomenon that serves as "a basis for inferring how C and N cycling in this forest may respond to elevated CO₂ beyond the decadal time scale."

"Using natural-abundance measures of nitrogen isotopes together with an ecosystem-scale ¹⁵N tracer experiment," as the six scientists describe it, they "quantified the cycling of ¹⁵N in plant and soil pools under ambient and elevated CO₂ over three growing seasons to determine how elevated CO₂ changed nitrogen cycling between plants, soil and microorganisms," after having first measured natural-abundances of ¹⁵N in plant and soil pools within the two CO₂ treatments over the prior year. And as a result of these efforts, they discovered that "at the Duke FACE site, the rate at which N is being sequestered in plant biomass is greater than the rate of atmospheric deposition and heterotrophic N fixation," which has also been established by the work of Finzi et al. (2002), Hofmockel and Schlesinger (2007) and Sparks et al. (2008), all of which findings suggest, in their words, that "soil organic matter decomposition supplies a significant fraction of plant N in both ambient and elevated-CO₂ conditions, but that this is greater under elevated CO₂."

Based on these real-world experimental observations, Hofmockel et al. conclude that "in pine forests of the southeastern United States, rising CO₂ may elicit shifts in the mechanisms by which plants acquire nitrogen, allowing a sustained increase in net primary productivity for decades," while further opining that "increased mineralization of nitrogen in the organic and 0-15 cm mineral horizon and deeper rooting are likely sustaining the elevated CO₂ enhancement of net primary productivity."

Additional References
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.

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.

Hofmockel, K.S. and Schlesinger, W.H. 2007. Carbon dioxide effects on heterotrophic dinitrogen fixation in a temperate pine forest. Soil Science Society of America Journal 71: 140-144.

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.

McCarthy, H,.R., Oren, R., Johnsen, K.H., Gallet-Budynek, A., Pritchard, S.G., Cook, C.W., LaDeau, S.L., Jackson, R.B. and Finzi, A.C. 2010. Re-assessment of plant carbon dynamics at the Duke free-air CO₂ enrichment site: interactions of atmospheric [CO₂] with nitrogen and water availability over stand development. New Phytologist 185: 514-528.

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.

Sparks, J.P., Walker, J., Turnipseed, A. and Guenther, A. 2008. Dry nitrogen deposition estimates over a forest experiencing free air CO₂ enrichment. Global Change Biology 14: 768-781.

Zak, D.R., Holmes, W.E. and Pregitzer, K.S. 2007. Atmospheric CO₂ and O₃ alter the flow of N-15 in developing forest ecosystems. Ecology 88: 2630-2639.