1.  Critical assumptions to test include: Is leaf litter chemistry affected by elevated [CO 2 ]? Does a change in litter chemistry in elevated [CO 2 ] alter decomposition?  Ecosystem models are used to explore the consequences for N cycling, primary productivity, and C storage of CO 2 effects on litter chemistry and decomposition.  A consensus is needed from experimental data on how to best represent these effects in models. The GCTE Synthesis of Litter Chemistry and Decomposition in Elevated CO 2 R. J. Norby 1, M. F. Cotrufo 2, P. Ineson 3, E. G. O’Neill 1, and J. G. Canadell 4 1 Oak Ridge National Laboratory, USA; 2 II Universitá di Napoli, Italy; 3 University of York, UK; 4 GCTE Project Office, Australia The Litter Quality Hypothesis  Plant tissue grown in elevated concentrations of atmospheric CO 2 is usually found to have lower concentrations of N than comparable tissue grown in ambient CO 2.  If plant detritus (e.g., leaf litter) also has lower [N] in CO 2 -enriched ecosystems, then the community of decomposing organisms will have food of lower quality, and the transfer of organically-bound N to mineral N pools available to support plant growth could slow.  Ecosystem models show that the accelerated growth of plants in high CO 2 atmospheres would be self- limited by this litter-quality feedback. Methods of Meta-Analysis  Litter chemistry data (N and lignin concentrations) were assembled from observations of naturally senseced leaves of plants exposed to elevated [CO 2 ] (600-700 ppm) in the field or in field chambers.  Decomposition data base (mass loss and CO 2 release) also included other plant tissues and experiments using different CO 2 enrichment regimes.  Categorical variables were defined to describe source material characteristics, exposure protocol, and measurement protocol.  The effect size for meta-analysis was lnRR where RR is the response ratio (mean in elevated [CO 2 ] divided by mean in ambient [CO 2 ]). A mixed model was used. GCTE Synthesis – Approach & Products  GCTE Focus 1 (Ecosystem Physiology) identified litter quality to be a critical response that could influence ecosystem response to rising CO 2.  A workshop “Litter Quality and Decomposition under Elevated CO 2 ” was held in Capri, Italy, in September, 1998, with the primary objective of reaching consensus on the experimental results.  Workshop products included a meeting report [Nature (1998) 396: 17-18] and a volume of research papers [Plant & Soil (2000) 224, No. 1].  A meta-analysis of all relevant published and pre- publication data on CO 2 effects on litter chemistry and decomposition was published in Oecologia (2001) 127: 153-165.  The workshop developed a set of recommendations for standardizing measures of litter quality and decomposition in ongoing and future CO 2 enrichment experiments. Litter NitrogenLitter LigninDecomposition Conclusions  The hypothesis that CO 2 -induced changes in leaf litter chemistry result in impacts on decomposition is not supported.  N concentration of naturally senesced leaves of plants grown in elevated [CO 2 ] was 7.1% lower than that of ambient-grown plants. This result was: usually not significant in individual experiments much less than that often observed in green leaves less in leaves with incomplete N resorption  The small, but consistent decline in litter [N], coupled with a 6.5% increase in lignin, would be predicted to result in slower decomposition in CO 2 -enriched litter. However, there was no consistent effect of CO 2 treatment on mass loss or CO 2 release.  CO 2 effects on litter chemistry or decomposition were smallest under experimental conditions more similar to field conditions.  Any changes in decomposition rate resulting from exposure of plants to elevated [CO 2 ] are small compared to other potential impacts of [CO 2 ] on C and N cycling. N Resorption CO 2 Release Symbols open – herbaceous closed – woody circles – OTCs squares – open-field triangles - solardomes Median of 67 observations: 9.8 mg/g in ambient 9.2 mg/g in elevated CO 2 Symbols open – herbaceous closed – woody circles – OTCs squares – open-field triangles - solardomes Mean of 20 observations: 49% in ambient 48% in elevated CO 2 Note: no significant effect of [CO 2 ] if 95% confidence interval includes a response ratio of 1. The number of observations is indicated. “no chamber” includes FACE, CO 2 springs, and SACC. Symbols open – herbaceous closed – woody Shapes represent different species Median of 46 observations: 138 mg/g in ambient 149 mg/g in elevated CO 2 Symbols open – herbaceous closed – woody circles – OTCs squares – open-field uptriangles – solardomes downtriangles – growth chambers Symbols open – herbaceous closed – woody RE = (green [N] –litter [N])/green [N] Note: the data set did not support meta-analysis Acknowledgements -- This synthesis was made possible with the financial support of the U.S. Department of Energy and the contributions of published and unpublished data from many researchers in the GCTE CO 2 network.