Background
Rhizodeposition, in the words of the authors, is "the addition of C [carbon] from roots to soil C pools," and they note that "because up to 80% of the biomass and at least 50% of net primary production can occur below-ground in grasslands, changes in rhizodeposition will have a large impact on C cycling in these ecosystems (Milchunas and Lauenroth, 2001)," which phenomenon may significantly impact the amount of carbon sequestered in grassland soils.

What was done
Pendall et al. used open-top chambers to study various responses of a native C3-C4 grassland ecosystem in the shortgrass steppe region of northeastern Colorado, USA, to a doubling of the air's CO2 concentration (from 360 to 720 ppm) in an experiment that lasted five years.

What was learned
Total aboveground biomass was increased by an average of 33% over the course of the study in the CO2-enriched chambers, while belowground biomass was increased by an average of 23%.  In addition, over the last four years of the experiment, rhizodeposition was increased by a whopping 137% in the chambers exposed to elevated CO2.  However, Pendall et al. report that "decomposition increased nearly as much as rhizodeposition," leading to little net increase in soil C storage in the CO2-enriched chambers relative to that experienced in the ambient-air chambers.

What it means
Although there was little increase in soil carbon sequestration in the CO2-enriched chambers of this particular study, more biomass was produced within them each year, both above- and below-ground, than in the ambient-air chambers; and much more biomass made its way into the soil of the CO2-enriched chambers.  Hence, it is likely that over the course of several decades, the small yearly differences in soil C storage would eventually end up producing a substantially larger stash of carbon in the soil of the CO2-enriched chambers, which is why it is so very important to continue these types of studies for much longer periods of time than is usually done.  See also, in this regard, our Editorial of 5 Mar 2003, which explains the merits of, and need for, much longer-than-typical studies of tree responses to atmospheric CO2 enrichment.

Reference
Milchunas, D. and Lauenroth, W.  2001.  Belowground primary production by carbon isotope decay and long-term root biomass dynamics.  Ecosystems 4: 139-150.