Palatability trials on hardwood leaf litter grown under elevated CO2: a stable carbon isotope study

The palatability to isopods and microbes of a broad range of hardwood leaf litter, derived from three field CO2-enrichment experiments in the USA, was investigated, using δ13C, to trace the C flow from litter to isopods and to CO2 respired by microbial decomposition. Leaf litter grown under elevated CO2 had δ13C values ranging from −39 to −45‰, which were significantly different from ambient litter δ13C values of around −30‰. Litter palatability to isopods of the Porcellio sp. was tested by incubating ambient- and elevated-CO2 litter, and a mixture of the two, in the presence of isopods for 14 days, under environmentally controlled conditions; δ13C was measured on litter and isopods' body before and after incubation. In an additional experiment, litter was incubated in the absence of fauna for 30 days, and on five occasions the δ13C of the CO2 respired from litter was measured. The 13C label was clearly carried from the litter source to the isopods' bodies, and their faeces. For microbial-respired CO2, δ13C was significantly higher than that of the litter source, suggesting preferential degradation of substrates enriched in 13C as compared to those in the overall litter. With the exception of Quercus myrtifolia leaf litter, elevated CO2 did not affect the palatability to isopods nor the microbial degradation of any of the litters, possibly as a result of unaltered litter N concentration. However, significant differences in litter palatability and decay rates were observed among the different species. With this study, the use of isotopically labelled litter material was confirmed as a key methodology that can significantly contribute to the advancement of the understanding of litter decomposition and of the quantification of C fluxes in the process.