Volume 15, Number 6: 8 February 2012
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 new study of 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."
Sherwood, Keith and Craig Idso
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., Gallet-Budynek, A., McCarthy, H.R., Currie, W.S., Jackson, R.B. and Finzi, A. 2011. Sources of increased N uptake in forest trees growing under elevated CO2: results of a large-scale 15N study. Global Change Biology 17: 3338-3350.
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.