Soil Carbon and Nitrogen in a CO₂-Enriched Sweetgum Plantation

In introducing their study, Iversen et al. (2012) 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)."

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).

In doing so, 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."

Given such results, 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."

Additional References
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.