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Effects of Elevated CO2 on Leaf Litter Quality and Decomposition in Mediterranean Forest Species
De Angelis, P., Chigwerewe, K.S. and Mugnozza, G.E.S.  2000.  Litter quality and decomposition in a CO2-enriched Mediterranean forest ecosystem.  Plant and Soil 224: 31-41.

What was done
Large open-top chambers were constructed around 30-year-old mixed stands of naturally growing Mediterranean forest species located near the coast of central Italy.  Half of the chambers were exposed to ambient air of 350 ppm atmospheric CO2 concentration, while half were exposed to air enriched to 710 ppm CO2 to determine the effects of elevated CO2 on leaf litter quality and its rate of decomposition.  The dominant woody species observed in this study were Quercus ilex, Phillyrea augustifolia, and Pistacia lentiscus.  Trees of these species had been subjected to differential CO2 exposure for between three and four years prior to the beginning the leaf litter quality and decomposition measurements.

What was learned
Elevated CO2 significantly altered the characteristics of leaf litter in all three woody species.  Lignin and carbon concentrations, for example, were increased by 18 and 4%, respectively, while nitrogen concentrations were reduced by 13%.  These changes resulted in a 20% CO2-induced increase in the carbon-to-nitrogen ratio of the leaf litter, which is a common parameter used to predict decomposition rates (larger ratios are generally associated with relatively less decomposition than smaller ratios).

In monitoring the decomposition of the leaf litter produced by the trees in the different CO2 treatments, it was found that the initial loss of mass from the litter was about 8% less in the CO2-enriched chambers than in the ambient-treatment chambers; and after one year, 4% less decomposition had occurred in the leaf litter gathered from beneath the CO2-enriched trees than in litter collected beneath the trees growing in ambient air.

What it means
As the CO2 content of the atmosphere continues to rise, it is likely that CO2-induced changes in leaf litter quality of Mediterranean forest species may result.  Such changes in litter quality will likely cause reductions in its decomposition rate.  These observations prompted the authors to conclude that, "if this effect is coupled to an increase of primary production [which nearly always occurs in response to elevated CO2] there will be a net rise of C-storage in the soils of forest ecosystems."  Thus, as the atmospheric CO2 concentration rises, the soils of Mediterranean forest ecosystems will likely increase their sequestration of carbon.

Reviewed 20 December 2000