How does rising atmospheric CO2 affect marine organisms?

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Decomposition (Grassland Species) -- Summary
Grassland species grown at elevated atmospheric CO2 concentrations nearly always exhibit increased photosynthetic rates and biomass production.  Due to this productivity enhancement, 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.  Such plant material, however, sometimes has a greater carbon-to-nitrogen ratio than plant material produced under ambient CO2 concentrations.  Thus, a question arises as to whether or not such changes in plant litter quality affect its decomposition rate.  And if a change in decomposition rate occurs, there is the subsequent question of how that phenomenon might impact biological carbon sequestration.  In searching for answers to these thought-provoking questions, we turn to the scientific literature to see what has recently been published in this field.

In the study of Nitschelm et al. (1997), white clover exposed to an atmospheric CO2 concentration of 600 ppm for one growing season channeled 50% more newly-fixed carbon compounds into the soil than similar plants exposed to ambient air.  In addition, the cloverís roots decomposed at a rate that was 24% slower than that observed for roots of control plants, as has also been reported for white clover by David et al. (2001).  These observations suggest that soil carbon sequestration under white clover ecosystems will be greatly enhanced as the airís CO2 content continues to rise, as was also shown for moderately fertile sandstone grasslands (Hu et al., 2001).

Similar results have been observed with mini-ecosystems comprised entirely of perennial ryegrass.  Van Ginkel et al. (1996), for example, demonstrated that exposing this species to an atmospheric CO2 concentration of 700 ppm for two months caused a 92% increase in root growth and 19 and 14% decreases in root decomposition rates one and two-years, respectively, after incubating ground roots within soils.  This work was later followed up by Van Ginkel and Gorissen (1998), who showed a 13% reduction in the decomposition rates of CO2-enriched perennial ryegrass roots in both disturbed and undisturbed root profiles.  This and other work led the authors to calculate that CO2-induced reductions in the decomposition of perennial ryegrass litter, which enhances soil carbon sequestration, could well be large enough to remove over half of the anthropogenic CO2 emissions that may be released in the next century (Van Ginkel et al., 1999).  Furthermore, Van Ginkel et al. (2000) determined that predicted increases in surface air temperature would not adversely impact the strength of this powerful biological carbon sink.  Indeed, after experimentally raising ambient air temperatures by 2 įC, they reported that CO2-induced reductions in litter decomposition rates were still 12% slower than those of litter grown in ambient air.  In fact, even a 6 įC increase in air temperature could not destroy the CO2-induced reductions in litter decomposition rates.

In some cases, atmospheric CO2 enrichment has little or no significant effects on litter quality and subsequent rates of litter decomposition, as was the case in the study of Hirschel et al. (1997) for lowland calcareous and high alpine grassland species.  Similar non-effects of elevated CO2 on litter decomposition have also been reported in a California grassland (Dukes and Field, 2000).

In light of these several experimental findings, it would appear that as the airís CO2 concentration increases, litter decomposition rates of grassland species will likely decline, increasing the amount of carbon sequestered in grassland soils.  Since this phenomenon is augmented by the aerial fertilization effect of atmospheric CO2 enrichment, which leads to the production of greater amounts of litter, there is thus a double reason for expecting more carbon to be removed from the atmosphere by earthís grasslands in the future.

References
David, J.-F., Malet, N., Couteaux, M.-M. and Roy, J.  2001.  Feeding rates of the woodlouse Armadillidium vulgare on herb litters produced at two levels of atmospheric CO2Oecologia 127: 343-349.

Dukes, J.S. and Field, C.B.  2000.  Diverse mechanisms for CO2 effects on grassland litter decomposition.  Global Change Biology 6: 145-154.

Hirschel, G., Korner, C. and Arnone III, J.A.  1997.  Will rising atmospheric CO2 affect leaf litter quality and in situ decomposition rates in native plant communities?  Oecologia 110: 387-392.

Hu, S., Chapin III, F.S., Firestone, M.K., Field, C.B. and Chiariello, N.R.  2001.  Nitrogen limitation of microbial decomposition in a grassland under elevated CO2Nature 409: 188-191.

Nitschelm, J.J., Luscher, A., Hartwig, 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.

Van Ginkel, J.H. and Gorissen, A.  1998.  In situ decomposition of grass roots as affected by elevated atmospheric carbon dioxide.  Soil Science Society of America Journal 62: 951-958.

Van Ginkel, J.H., Gorissen, A. and Polci, D.  2000.  Elevated atmospheric carbon dioxide concentration: effects of increased carbon input in a Lolium perenne soil on microorganisms and decomposition.  Soil Biology & Biochemistry 32: 449-456.

Van Ginkel, J.H., Whitmore, A.P. and Gorissen, A.  1999.  Lolium perenne grasslands may function as a sink for atmospheric carbon dioxide.  Journal of Environmental Quality 28: 1580-1584.

Van Ginkel, J.H., Gorissen, A. and van Veen, J.A.  1996.  Long-term decomposition of grass roots as affected by elevated atmospheric carbon dioxide.  Journal of Environmental Quality 25: 1122-1128.