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Carbon Sequestration in a Tallgrass Prairie Soil
Williams, M.A., Rice, C.W., Omay, A. and Owensby, C. 2004. Carbon and nitrogen pools in a tallgrass prairie soil under elevated carbon dioxide. Soil Science Society of America Journal 68: 148-153.

The authors write that "soil is a potential carbon sink and could offset rising atmospheric CO2." Hence, it is important to determine how the ongoing rise in the air's CO2 content may be impacting this phenomenon.

What was done
Over the course of an eight-year open-top-chamber CO2-enrichment (to twice ambient concentrations) study of a pristine (annually burned) tallgrass prairie north of Manhattan, Kansas, USA, which was composed of a mixture of C3 and C4 species, Williams et al. measured changes in the active, slow and passive pools of carbon (C) and nitrogen (N), with the goal of determining how they were impacted by the doubled atmospheric CO2 concentration of their reasonably long-term experiment.

What was learned
The researchers report that "on average, elevated CO2 induced a 60% increase in root growth," citing Owensby et al. (1999). They also found that potentially mineralizable C was enhanced by 19% and 24%, respectively, in the 0- to 5-cm and 5- to 15-cm soil horizons, which they suggest implies that "increases in plant inputs have outpaced increases in decomposition rates" in those layers. Likewise, they report that potentially mineralizable N was enhanced by 14% in the 0- to 5-cm layer, and that CO2 enrichment resulted in greater recalcitrant N in the 5- to 15-cm soil layer. With respect to these latter findings, they hypothesize that "greater N translocated to aboveground biomass from deeper soil depths could ultimately be stored in roots and rhizomes near the soil surface, and would eventually turnover and become a part of the surface soil N pool."

What it means
Williams et al. conclude, in their words, that "the 60% increase in root growth during the 8-year study was the likely catalyst for the greater potentially mineralizable soil C pools in the enriched CO2 treatment," and that this finding "confirms that C can accrue in soils under elevated CO2." In their specific study, for example, they determined that the total amount of extra new carbon sequestered in the soil due to their doubling of the air's CO2 concentration was 4 Mg C ha-1 over the 8-year period, for an annual rate of extra (CO2-induced) carbon sequestration of 0.5 Mg ha-1 year-1.

Reviewed 22 February 2006