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Do Earth's Soils Contain Enough Nitrogen for Its Plants to Receive the Full Benefits of Atmospheric CO2 Enrichment?
Volume 12, Number 43: 28 October 2009

In the very first sentence of the abstract of a recently-published paper that has been touted by climate alarmists as portending even greater global warming than the massive amount already being predicted by state-of-the-art climate models, Thornton et al. (2009) write that "inclusion of fundamental ecological interactions between carbon and nitrogen cycles in the land component of an atmosphere-ocean general circulation model leads to decreased carbon uptake associated with CO2 fertilization," and they claim that this phenomenon reduces "the fraction of anthropogenic CO2 predicted to be sequestered in land ecosystems," which indeed it would do ... if the concept was true.

In justifying the use of their new model component, the ten scientists (hailing from nine different research institutions) state that the results they obtained with it are "consistent with the progressive nitrogen limitation hypothesis," which sounds good ... But is it?

As indicated in our Editorial of 21 October 2009, the progressive nitrogen limitation hypothesis - as logical as it may sound - is not consistent with real-world experimental data; and as we go on to report here, one of the best refutations of the hypothesis has been provided by work conducted by researchers at the very same institution where Thornton works - the Environmental Sciences Division of the Oak Ridge National Laboratory (ORNL) located in Oak Ridge, Tennessee, USA.

There, at ORNL's Free-Air CO2 Enrichment or FACE facility, sweetgum (Liquidambar styraciflua) trees had been grown out-of-doors in a low-nitrogen soil for ten years in normal ambient air and then divided into two groups, one of which was allowed to continue to grow in ambient air and one of which was exposed to an extra 150 ppm of CO2 during daylight hours for ten additional years. Then, near the end of this period - in mid-May of the ninth year of differential CO2 treatment - Garten and Brice (2009) injected two clusters of three nearest-neighbor trees in each FACE plot of the two CO2 treatments with 3.2 g of 15N-ammonium sulfate, after which soil and fine-root samples were collected beneath the trees at 14, 34, 49, 66 and 89 weeks after the time of injection.

Various analyses of the data they obtained led Garten and Brice to conclude that over the course of their study, "the fate of 15N beneath labeled sweetgum trees did not support a hypothesis of progressive nitrogen limitation [italics added]." Indeed, they found that "the absence of a difference in belowground 15N dynamics between ambient and elevated CO2 and the absence of greater association of 15N with more stable forms of soil organic matter under elevated CO2 both indicated that progressive nitrogen limitation of net primary production was not at work [italics added]."

For good measure, the two researchers also note that "prior studies of aboveground (Norby and Iversen, 2006) and belowground (Zak et al., 2003) process level responses to elevated CO2 are consistent with our 15N tracer study, indicating that measurable progressive nitrogen limitation is not underway at the ORNL FACE Experiment [italics added]." Nor, we might add, has it been detected in any of the 26 other studies of the putative phenomenon that we have reviewed on our website: see Nitrogen (Progressive Limitation Hypothesis) in our Subject Index. Thus, wherever competent researchers have looked, the hypothesized progressive nitrogen limitation of the aerial fertilization effect of atmospheric CO2 enrichment has yet to be found, probably because it simply doesn't occur in the real world.

Sherwood, Keith and Craig Idso

Garten Jr., C.T. and Brice, D.J. 2009. Belowground fate of 15N injected into sweetgum trees (Liquidambar styraciflua) at the ORNL FACE Experiment. Rapid Communications in Mass Spectrometry 23: 3094-3100.

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.

Thornton, P.E., Doney, S.C., Lindsay, K., Moore, J.K., Mahowald, N., Randerson, J.T., Fung, I., Lamarque, J.-F., Feddema, J.J. and Lee, Y.-H. 2009. Carbon-nitrogen interactions regulate climate-carbon cycle feedbacks: results from an atmosphere-ocean general circulation model. Biogeosciences 6: 2120-2120.

Zak, D.R., Holmes, W.E., Finzi, A.C., Norby, R.J. and Schlesinger, W.H. 2003. Soil nitrogen cycling under elevated CO2: A synthesis of forest FACE experiments. Ecological Applications 13: 1508-1514.