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Grasslands (Decomposition) -- Summary
As the CO2 content of the air rises, many plants may respond by reducing their foliar nitrogen contents, as they mobilize nitrogen away from more efficiently operating photosynthetic components and into areas where other physiological processes may be lacking this essential element.  This phenomenon sometimes, but not always, results in leaf litter that has a lower nitrogen content than litter derived from ambiently-grown plants.  In addition, CO2-induced increases in photosynthetic rates often lead to greater carbohydrate production, which can enhance the concentrations of carbon compounds in various plant organs.  Following plant death or organ senescence, therefore, higher concentrations of carbon compounds can sometimes be found in the litter of CO2-enriched plants than in the litter produced by plants grown in ambient air.  This being the case, it is important to determine how elevated levels of atmospheric CO2 impact the decomposition of chemically-altered plant litter produced under conditions of atmospheric CO2 enrichment, which is the subject of this brief literature review that concentrates on the plants of various grassland ecosystems.

In two of the studies discussed on our website, few significant effects of elevated CO2 on the decomposition of plant litter were identified.  Hirschel et al. (1997) took fallen leaves from species common to alpine and calcareous grasslands and incubated them in mesh nylon bags placed on the soil surfaces of their respective mesocosms, which were subjected to ambient and twice-ambient concentrations of atmospheric CO2.  After one year of incubation, they determined that the decomposition rates of the litter produced under the ambient and elevated CO2 conditions did not differ significantly from each other, except in the case of one alpine sedge, which exhibited reduced rates of decomposition under conditions of atmospheric CO2 enrichment.  Similar results were reported by Dukes and Field (2000), who incubated litter derived from various combinations of grassland species common to California, USA, for an eight-month period at atmospheric CO2 concentrations of 350 and 700 ppm and reported minimal effects of elevated CO2 on decomposition rates.

In two other studies, both research teams identified significant effects of elevated CO2 on decomposition rates of litter generated from CO2-enriched grassland plants.  In the study of van Ginkel and Gorissen (1998), disturbed and undisturbed root-soil systems from swards of perennial ryegrass subjected to ambient and elevated atmospheric CO2 concentrations for three months were allowed to decompose in sealed containers for an eight-month period.  The researchers then determined that the decomposition rates of disturbed root-soil systems produced under an atmospheric CO2 concentration of 700 ppm were 13% lower than the rates of control root-soil systems generated under ambient CO2 concentrations.  In addition, they found that the decomposition rates of CO2-enriched undisturbed root-soil systems were only slightly higher than rates observed for CO2-enriched disturbed systems; but they were still significantly less than rates calculated for ambient controls.

In the last of the studies we have reviewed on our website, that of van Ginkel et al. (2000), it was shown that elevated CO2 decreased root decomposition rates of perennial ryegrass grown at 700 ppm CO2 for 115 days by 14% after 230 days of incubation in elevated CO2.  In addition, the scientists determined that raising the incubation temperature by 2 įC had little effect on the CO2-induced reductions in decomposition rate, for they were still 12% lower than those measured at 350 ppm CO2.  In fact, in a shorter-term experiment the researchers discovered that even a 6 įC increase in air temperature could not counterbalance the CO2-induced reductions in decomposition rate.

These results suggest that future increases in the airís CO2 content will likely cause slight reductions in rates of plant litter decomposition, thus allowing more carbon to remain sequestered from the atmosphere for longer periods of time, and that this will be the case even if earthís air temperature were to rise for some reason.

References
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.

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. 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.