The Progressive Nitrogen Limitation Hypothesis Takes Another Hit

According to authors Langley et al., "it has been suggested that stimulation of productivity with elevated CO₂ ties up nitrogen in plant litter, which, if not offset by increases in N-use efficiency or N supply, will limit the ecosystem CO₂ response (Reich et al., 2006)." In an effort to test this hypothesis, the authors "employed an acid-hydrolysis-incubation method and a net nitrogen-mineralization assay to assess stability of soil carbon pools and short-term nitrogen dynamics in a Florida scrub-oak ecosystem after six years of exposure to elevated CO₂," which work was conducted at a multiple open-top-chamber facility on a barrier island located at NASA's Kennedy Space Center on the east coast of central Florida, USA. So what did they learn?

The six researchers report that elevated atmospheric CO₂ (to 350 ppm above ambient concentrations) tended to increase net N mineralization in the top 10 cm of the soil, but that it also decreased total soil organic carbon content there by 21%. However, that loss of carbon mass was only equivalent to "roughly one-third of the increase in plant biomass that occurred in the same experiment." In addition, they state that the strongest increases in net N mineralization were observed in the 10-30 cm depth increment, and that "release of N from this depth may have allowed the sustained CO₂ effect on productivity in this scrub-oak forest," which over the four years leading up to their study "increased litterfall by 19-59%," for which latter figures they cite the work of Hungate et al. (2006).

Once again, we have yet another experimental demonstration of the fact that atmospheric CO₂ enrichment generally enables plants to find the extra nitrogen they need to take full advantage of the aerial fertilization effect of elevated atmospheric CO₂ concentrations, with the result that total ecosystem carbon content is increased, resulting in a negative feedback to anthropogenic CO₂ emissions.

Additional References
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.

Reich, P.B., Hungate, B.A. and Luo, Y. 2006. Carbon-nitrogen interactions in terrestrial ecosystems in response to rising atmospheric carbon dioxide. Annual Review of Ecology, Evolution and Systematics 37: 611-636.