Elevated CO2, litter chemistry, and decomposition: a synthesis

The results of published and unpublished experiments investigating the impa cts of elevated [CO2] on the chemistry of leaf litter and decomposition of plant tissues are summarized. The data do not support the hypothesis that c hanges in leaf Litter chemistry often associated with growing plants under elevated [CO2] have an impact on decomposition processes. A meta-analysis o f data from naturally senesced leaves in field experiments showed that the nitrogen (N) concentration in leaf litter was 7.1% lower in elevated [CO2] compared to that in ambient [CO2]. This statistically significant differenc e was: (1) usually not significant in individual experiments, (2) much less than that often observed in green leaves, and (3) less in leaves with an N concentration indicative of complete N resorption. Under ideal conditions, the efficiency with which N is resorbed during leaf senescence was found n ot to be altered by CO2 enrichment, but other environmental influences on r esorption inevitably increase the variability in litter N concentration. Ne vertheless, the small but consistent decline in leaf litter N concentration in many experiments, coupled with a 6.5% increase in lignin concentration, would be predicted to result in a slower decomposition rate in CO2-enriche d lifter. However, across the assembled data base, neither mass loss nor re spiration rates from litter produced in elevated [CO2] showed any consisten t pattern or differences from litter grown in ambient [CO2]. The effects of [CO2] on litter chemistry or decomposition were usually smallest under exp erimental conditions similar to natural field conditions, including open-fi eld exposure, plants free-rooted in the ground, and complete senescence. It is concluded that any changes in decomposition rates resulting from exposu re of plants to elevated [CO2] are small when compared to other potential i mpacts of elevated [CO2] on carbon and N cycling. Reasons for experimental differences are considered, and recommendations for the design and executio n of decomposition experiments using materials from CO2-enrichment experime nts are outlined.