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Does Atmospheric CO2 Enrichment Alter Leaf Litter Chemistry and Decomposition?
Norby, R.J., Cotrufo, M.F., Ineson, P., O'Neill, E.G. and Canadell, J.G.  2001.  Elevated CO2, litter chemistry, and decomposition: a synthesis.  Oecologia 127: 153-165.

What was done
The authors conducted a meta-analysis of the results obtained from several dozen studies of the effects of elevated atmospheric CO2 concentration on leaf litter chemistry and decomposition rate.

What was learned
Based on a total of 67 experimental observations, the authors found that elevated atmospheric CO2 concentrations, mostly between 600 and 700 ppm, reduced leaf litter nitrogen concentration by an average of approximately 7%.  But in experiments where plants were grown under as close to natural conditions as possible, such as in
open-top chambers, FACE plots or in the proximity of CO2-emitting springs, there were no significant effects of elevated CO2 on leaf litter nitrogen content.

In addition, based on a total of 46 experimental observations, the authors determined that elevated atmospheric CO2 concentrations increased leaf litter lignin concentrations by an average of about 6.5%.  However, these increases in lignin content occurred only in woody, and not herbaceous, species.  And again, lignin concentrations of leaf litter were not affected by elevated CO2 when plants were grown in open-top chambers, FACE plots or in the proximity of CO2-emitting springs.

Finally, in analyzing a total of 101 observations, the authors found that elevated CO2 had no consistent effect on leaf litter decomposition rate in any type of experimental setting.

What it means
As the air's CO2 content rises, it will likely have little to no impact on leaf litter chemistry and subsequent rates of leaf litter decomposition.