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The Progressive Nitrogen Limitation Hypothesis
Reference
Finzi, A.C., Norby, R.J., Calfapietra, C., Gallet-Budynek, A., Gielen, B., Holmes, W.E., Hoosbeek, M.R., Iversen, C.M., Jackson, R.B., Kubiske, M.E., Ledford, J., Liberloo, M., Oren, R., Polle, A., Pritchard, S., Zak, D.R., Schlesinger, W.H. and Ceulemans, R. 2007. Increases in nitrogen uptake rather than nitrogen-use efficiency support higher rates of temperate forest productivity under elevated CO2. Proceedings of the National Academy of Sciences, USA 104: 14,014-14,019.

Background
The authors write that "in nitrogen-limited ecosystems, the rate at which N is converted to available forms is slow relative to the rate of N uptake by trees, and as a consequence it is assumed there is little or no additional capacity of soils to supply N to forest trees." Hence, it has long been claimed by many -- but ever disbelieved by us -- that as trees growing at accelerated rates in CO2-enriched air begin to need increased quantities of nitrogen to maintain those elevated growth rates, they will not find it in N-limited soils and their rates of growth will thus return to "normal" or even something less. This concept is referred to as the progressive nitrogen limitation hypothesis, and it has been invoked again and again by people who refuse to believe that the ongoing rise in the air's CO2 content has the power to raise plant productivity the world over to levels never before experienced in human history.

What was done
Finzi et al. evaluated this concept based on data obtained from four well-known free-air CO2-enrichment (FACE) experiments conducted on forests -- the Rhinelander, Duke and Oak Ridge National Laboratory (ORNL) studies in the United States, and the POP-EUROFACE study in Europe -- where previous research described by Norby et al. (2005) showed that net primary production (NPP) increased by 23 2% in response to a CO2 concentration increase of 174 ppm (46%) above the mean ambient-air concentration.

What was learned
The CO2-induced increase in forest productivity at the POP-EUROFACE site, which they say was "located on former agricultural land where soil nitrogen availability was high and not limiting," was found by them to not have been supported by greater nitrogen (N) uptake from the soil, but by an increase in nitrogen use efficiency (NUE). At the other three sites, however, the CO2-induced increase in forest productivity was supported by greater N uptake from the soil, with no change in NUE; and they note that this result was "unexpected," especially for the Duke and ORNL sites, where they say that "tree growth is demonstrably N-limited."

What it means
Focusing on the findings of the three U.S. studies, Finzi et al. state that "the response of N uptake and NUE in these young temperate forests exposed to FACE is the opposite [our italics] of that predicted by the current generation of biogeochemical models," i.e., those that are based upon the progressive nitrogen limitation hypothesis; and after discussing some possible ways by which these forests might be obtaining the seemingly-impossible-to-obtain nitrogen that they need to maintain their significantly-CO2-enhanced growth rates, they conclude by stating that "regardless of the specific mechanism, this analysis demonstrates that larger quantities of carbon entering the below-ground system under elevated CO2 result in greater N uptake, even in N-limited ecosystems." Hence, they further conclude that "biogeochemical models must be reformulated to allow carbon transfers below ground that result in additional N uptake under elevated CO2," which, of course, is what supports the ever-increasing plant growth rates that are fueling the ever-accelerating greening of the earth.

Reference
Norby, R.J., DeLucia, E.H., Gielen, B., Calfapietra, C., Giardina, C.P., King, S.J., Ledford, J., McCarthy, H.R., Moore, D.J.P., Ceulemans, R., De Angelis, P., Finzi, A.C., Karnosky, D.F., Kubiske, M.E., Lukac, M., Pregitzer, K.S., Scarasci-Mugnozza, G.E., Schlesinger, W.H. and Oren, R. 2005. Forest response to elevated CO2 is conserved across a broad range of productivity. Proceedings of the National Academy of Sciences 102: 10.1073/pnas.0509478102.

Reviewed 31 October 2007