Agricultural and forage plants grown in elevated CO2 concentrations nearly always exhibit increased photosynthetic rates and biomass production. Because of this greater growth, more plant material is typically added to soils from root growth, turnover, and exudation, as well as from leaves and stems following their abscission during senescence. Consequently, these phenomena tend to increase soil carbon contents in CO2-enriched atmospheres.
In an open-top chamber experiment conducted in Switzerland, for example, Nitschelm et al. (1997) reported that a 71% increase in the atmospheric CO2 concentration above white clover monocultures led to a 50% increase in soil organic carbon content. Related studies on wheat and soybean agroecosystems (Islam et al., 1999) provided similar results, as did a FACE experiment on cotton, which documented a 10% increase in soil organic carbon content in plots receiving 550 ppm CO2 relative to those receiving 370 ppm (Leavitt et al., 1994).
Atmospheric CO2 enrichment not only increases the amount of carbon entering soils, but it often maintains (Henning et al., 1996), or even increases, its residency time within that significant carbon pool. Torbert et al. (1998), for example, demonstrated that decomposition rates of CO2-enriched sorghum and soybean plant litter were significantly less than those measured for control litter produced at ambient CO2 concentration, as did Nitschelm et al. (1997), who reported a 24% decrease in the decomposition of CO2-enriched white clover roots. Furthermore, after two months of decomposition in the experiment of Torbert et al. (1998), 40% less carbon had evolved from soils containing CO2-enriched plant litter.
In summary, these studies demonstrate that atmospheric CO2 enrichment will likely increase soil carbon inputs while either maintaining or decreasing CO2 efflux from agricultural soils. Thus, as the CO2 content of the air increases, these two interacting phenomena will likely increase the carbon sequestering abilities of earth's agroecosystems. Moreover, as the organic carbon contents of agricultural soils increase, soil structure and quality will simultaneously be improved, which should enhance plant growth even more, leading to additional carbon sequestration under CO2-enriched conditions (Rasmussen et al., 1998; Islam et al., 1999).
References
Henning, F.P., Wood, C.W., Rogers, H.H., Runion, G.B. and Prior, S.A. 1996. Composition and decomposition of soybean and sorghum tissues grown under elevated atmospheric carbon dioxide. Journal of Environmental Quality 25: 822-827.
Islam, K.R., Mulchi, C.L. and Ali, A.A. 1999. Tropospheric carbon dioxide or ozone enrichments and moisture effects on soil organic carbon quality. Journal of Environmental Quality 28: 1629-1636.
Leavitt, S.W., Paul, E.A., Kimball, B.A., Hendrey, G.R., Mauney, J.R., Rauschkolb, R., Rogers, H., Nagy, J., Pinter Jr., P.J. and Johnson, H.B. 1994. Carbon isotope dynamics of free-air CO2-enriched cotton and soils. Agricultural and Forest Meteorology 70: 87-101.
Nitschelm, J.J., Luscher, A., Hatrwig, U.A. and van Kessel, C. 1997. Using stable isotopes to determine soil carbon input differences under ambient and elevated atmospheric CO2 conditions. Global Change Biology 3: 411-416.
Rasmussen, P.E., Goulding, K.W.T., Brown, J.R., Grace, P.R., Janzen, H.H. and Korschens, M. 1998. Long-term agroecosystem experiments: assessing agricultural sustainability and global change. Science 282: 893-896.
Torbert, H.A., Prior, S.A., Rogers, H.H. and Runion, G.B. 1998. Crop residue decomposition as affected by growth under elevated atmospheric CO2. Soil Science 163: 412-419.