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CO2 Effects on Carbon and Nitrogen in a Sweetgum Forest
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.

In the introduction to a Special Feature section of Ecology, Luo et al. (2006) write that the progressive nitrogen limitation (PNL) hypothesis "states that N sequestration in long-term organic matter pools will, without new N input and/or decreases in N losses, lead to a decline in mineral N availability over time at elevated CO2 compared to ambient CO2," with the result that there will be insufficient N to support long-term growth stimulation and carbon sequestration by elevated levels of atmospheric CO2. This hypothesis has been used by climate alarmists as a basis for claiming that the aerial fertilization effect of the ongoing rise in the air's CO2 content carries within it the seeds of its own destruction. However, a vast and growing body of scientific literature suggests that this hypothesis is incorrect, and that instead of low N constraining carbon cycling, enhanced carbon input stimulates nitrogen cycling, which tends to support the greening of the earth scenario that we have long championed.

What was done
In the first of Ecology's Special Feature papers to address the PNL hypothesis, Norby and Iversen (2006) describe what has been learned about the concept from "a six-year record of N dynamics of a sweetgum (Liquidambar styraciflua) stand exposed to elevated CO2 in the free-air CO2 enrichment (FACE) experiment at Oak Ridge, Tennessee, USA," which experiment has been the subject of several Journal Reviews on our website (see FACE Experiments (Trees - Sweetgum) in our Subject Index). More specifically, they say that they "evaluate the concept of PNL for this ecosystem from the perspective of N uptake, content, distribution, turnover, and N-use efficiency."

What was learned
In the words of the researchers, "net primary productivity in this stand has been significantly higher in CO2-enriched plots, and the response has been sustained through time, thereby meeting one of the criteria for the development of PNL." However, as they report, "none [our italics] of the [five] measured responses of plant N dynamics in this ecosystem indicated the occurrence of PNL."

What it means
To date, in what Norby and Iversen describe as "the longest observation period for N dynamics in a non-expanding deciduous forest exposed to elevated CO2," there is no indication of nitrogen limiting, or even beginning to limit, the ability of atmospheric CO2 enrichment to enhance tree productivity.

Luo, Y., Field, C.B. and Jackson, R.B. 2006. Does nitrogen constrain carbon cycling, or does carbon input stimulate nitrogen cycling? Ecology 87: 3-4.

Reviewed 19 April 2006