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Growth Enhancement Due to Elevated CO2 in a Semi-Arid Grassland
Dijkstra, F.A., Pendall, E., Mosier, A.R., King, J.Y., Milchunas, D.G. and Morgan, J.A. 2008. Long-term enhancement of N availability and plant growth under elevated CO2 in a semi-arid grassland. Functional Ecology 22: 975-982.

The authors write that "increased plant productivity and biomass accumulation with elevated CO2 is expected [but only by some, we hasten to add] to be constrained by soil N availability" and "even could result in progressive nitrogen limitation [our italics]," which hypothesis is discussed in our reviews of pertinent studies that we have archived under the general heading of Nitrogen (Progressive Limitation Hypothesis) in our Subject Index.

What was done
Dijkstra et al. employed open-top chambers to examine the effects of elevated atmospheric CO2 concentration (720 vs. 368 ppm) on nitrogen dynamics in a semi-arid grassland ecosystem in northeastern Colorado (USA), where they studied the impacts of elevated CO2 on N mineralization and plant N uptake by tracking initially-applied 15N and total N in both plants and soil over a period of five years.

What was learned
The six scientists report that enhanced plant production -- an increase in aboveground biomass on the order of 40% in response to their specific degree of elevated CO2 -- "did not lead to a progressive decline in soil N availability." In fact, they say that on the contrary, "soil N availability remained higher [our italics] after 5 years of elevated than ambient CO2," likely due to "a greater mineralization rate under elevated CO2." As for why this was so, they speculate that "elevated CO2 increased soil moisture due to decreased plant transpiration at our site (Nelson et al., 2004), which could have stimulated microbial activity and N mineralization."

What it means
Once again, we have another example of the failure of real-world data to provide any support for the progressive down-sizing of the aerial fertilization effect of atmospheric CO2 enrichment due to a progressive reduction in the availability of soil nitrogen, which flawed concept seems destined to slip quietly away into the fading sunset of unproven hypotheses.

Nelson, J.A., Morgan, J.A., LeCain, D.R., Mosier, A., Milchunas, D.G. and Parton, B.A. 2004. Elevated CO2 increases soil moisture and enhances plant water relations in a long-term field study in semi-arid shortgrass steppe of Colorado. Plant and Soil 259: 169-179.

Reviewed 14 January 2009