Abstract, Global Change Open Science Conference, Amsterdam, the Netherlands, July, 2001.
The GCTE Synthesis of Litter Chemistry and Decomposition in Elevated CO2
R. J. Norby*, M. F. Cotrufo, P. Ineson, E. G. O’Neill, J. G. Canadell
A prominent hypothesis concerning the response of terrestrial ecosystems to an increasing atmospheric CO2 concentration states that the detritus (specifically, leaf litter) of CO2-enriched plants will have a lower concentration of nitrogen (or higher C-to-N ratio) than that of plants in today’s atmosphere, and this change will cause decomposition rates to slow. Decomposition rate, relative to the rate of litter production, is a determinant of ecosystem C sequestration and a controller of N availability. The extent to which the C:N ratio of plant tissues can change is one of the key biogeochemical determinants regulating the amount of C that can be sequestered from the atmosphere into vegetation. Litter chemistry is, therefore, an important parameter in biogeochemical models of ecosystem response to atmospheric and climatic change.
The effects of elevated [CO2] on litter chemistry is a good example of how a physiological response that can be measured in a relatively short-term experiment can have longer-term ecological implications. Unfortunately, the experimental results on this response have been mixed, and the guidance provided to modelers is ambiguous. In light of the uncertainty, GCTE Focus 1 sponsored a workshop in September, 1998, to attempt to reach a consensus on this important topic, and a post-workshop meta-analysis of all relevant experimental data on the impacts of elevated [CO2] on the chemistry of leaf litter and decomposition of plant tissues was conducted.
The data from published and unpublished reports do not support the hypothesis that changes in leaf litter chemistry often associated with growing plants under elevated [CO2] result in impacts on decomposition processes. A meta-analysis of data from naturally senesced leaves in field experiments showed that the N concentration in leaf litter was 7.1% lower in elevated [CO2] compared to that in ambient [CO2]. This statistically significant difference was (i) usually not significant in individual experiments, (ii) much less than that often observed in green leaves, and (iii) less in leaves with an N concentration indicative of complete N resorption. Under ideal conditions, the efficiency by which N is resorbed during leaf senescence was found not to be altered by CO2 enrichment, but other environmental influences on resorption inevitably increase the variability in litter N concentration. Nevertheless, 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-enriched litter. However, across the assembled data base, neither mass loss nor respiration rates from litter produced in elevated [CO2] showed any consistent pattern or differences from litter grown in ambient [CO2]. The effects of [CO2] on litter chemistry or decomposition usually were smallest under experimental conditions more similar to natural field conditions, including open-field exposure, plants free-rooted in the ground, and complete senescence. It is concluded that any changes in decomposition rates resulting from exposure of plants to elevated [CO2] are small when compared to other potential impacts of elevated [CO2] on carbon and nitrogen cycling.