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Soil Carbon and Nitrogen in a CO2-Enriched Sweetgum Plantation
Iversen, C.M., Keller, J.K., Garten Jr., C.T. and Norby, R.J. 2012. Soil carbon and nitrogen cycling and storage throughout the soil profile in a sweetgum plantation after 11 years of CO2-enrichment. Global Change Biology 18: 1684-1697.

In introducing their study of the subject, the authors write that "the implications of deeper rooting distributions for future forest responses to rising atmospheric CO2 remain uncertain because deeper soil processes are poorly represented in ecosystem and land surface models (Iversen, 2010,) in part because relatively few data exist for model parameterization and testing (Rumpel and Kogel-Knabner, 2011)."

What was done
In an effort to help provide some experimental guidance in this area, Iversen et al. "explored whether greater C [carbon] and N [nitrogen] inputs from increased fine-root production and mortality in a CO2-enriched [to 565 ppm] forest plantation affect C and N cycling throughout the soil profile," which they did within the context of the Oak Ridge National Laboratory (ORNL) Free-Air CO2 Enrichment (FACE) experiment that was conducted in a Liquidambar styraciflua (sweetgum) plantation in eastern Tennessee (USA).

What was learned
Iversen et al. determined that (1) "greater inputs of fine roots resulted in the incorporation of new C into root-derived particulate organic matter pools to 90-cm depth," that (2) "even though production in the sweetgum stand was limited by soil N availability, soil C and N contents were greater throughout the soil profile under elevated CO2 at the conclusion of the experiment," that (3) "greater C inputs from fine-root detritus under elevated CO2 did not result in increased net N immobilization or C mineralization rates," and that (4) there was no indication of "significant priming of the decomposition of pre-experiment soil organic matter."

What it means
The four U.S. researchers conclude that "taken together with an increased mean residence time of C in deeper soil pools, these findings indicate that C inputs from relatively deep roots under elevated CO2 may increase the potential for long-term soil C storage," and that "expanded representation of biogeochemical cycling throughout the soil profile may improve model projections of future forest responses to rising atmospheric CO2."

Iversen, C.M. 2010. Digging deeper: fine-root responses to rising atmospheric CO2 concentration in forested ecosystems. New Phytologist 186: 346-357.

Rumpel, C. and Kogel-Knabner, I. 2011. Deep soil organic matter - a key but poorly understood component of terrestrial C cycle. Plant and Soil 338: 143-158.

Reviewed 3 October 2012