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CO2 Enrichment of a Scrub-Oak Woodland Low in Nitrogen
McKinley, D.C., Romero, J.C., Hungate, B.A., Drake, B.G. and Megonigal, J.P. 2009. Does deep soil N availability sustain long-term ecosystem responses to elevated CO2? Global Change Biology 15: 2035-2048.

It has periodically been suggested, in the words of the authors, that "the net transfer of labile or available nitrogen pools in soil to longer-lived plant and soil pools over time may reduce plant available forms of nitrogen in the soil," and that progressive nitrogen limitation (PNL) "would be a result of such a plant-soil nutrient feedback loop." It has also been suggested that this decreased N availability "could constrain plant and whole ecosystem responses to increased atmospheric CO2 concentrations." However, long-term studies of various ecosystems have shown this hypothesis to be incorrect, one of them being a study of the scrub-oak ecosystem of Merritt Island, which is home to NASA's Kennedy Space Center on the east coast of central Florida (USA), where for a period of eleven years the ecosystem has experienced a sustained accumulation of aboveground biomass in open-top chambers supplied with air enriched to 350 ppm CO2 above the ambient CO2 concentration.

What was done
McKinley et al. employed several assays performed in the sixth and eleventh years of this experiment that were designed to assess soil N dynamics and availability throughout the entire soil profile of the study site.

What was learned
The five researchers say that after eleven years of CO2 fumigation, they found "few changes in potential plant available N supply at elevated CO2 that would lead to PNL of plant responses." Instead, they report finding "slightly enhanced gross N mineralization in the shallower portions of the soil profile," which suggests, in their words, that "soil organic matter decomposition may be enhanced by elevated CO2, resulting in greater plant available N." In addition, they say they "found indirect evidence for greater plant N uptake or exploitation of inorganic N pools at elevated CO2 in deep portions of the soil profile." And they report they determined that "the water table could be a relatively large source of plant-available N," as well as "direct evidence that plants are taking up inorganic N from the water table."

What it means
McKinley et al. conclude that "sustained net primary production in the Florida scrub-oak ecosystem is likely a result of increased N availability, resulting from both increased N mineralization in shallow soils and great exploitation of deep soil N, including N uptake from the water table," thereby demonstrating once again -- see Nitrogen (Progressive Limitation Hypothesis) in our Subject Index -- that nature always seems to find a way to avoid the long-term negative consequences of the now-discredited PNL hypothesis.

Reviewed 28 October 2009