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要旨

During microbial breakdown of leaf litter a fraction of the C lost by the litter is not released to the atmosphere as CO₂ but remains in the soil as microbial byproducts. The amount of this fraction and the factors influencing its size are not yet clearly known. We performed a laboratory experiment to quantify the flow of C from decaying litter into the soil, by means of stable C isotopes, and tested its dependence on litter chemical properties. Three sets of C-13-depleted leaf litter (Liquidambar styraciflua L., Cercis canadensis L. and Pinus taeda L.) were incubated in the laboratory in jars containing C-13-enriched soil (i.e. formed C₄ vegetation). Four jars containing soil only were used as a control. Litter chemical properties were measured using thermogravimetry (Tg) and pyrolysis-gas chromatography/mass spectrometry-combustion interface-isotope ratio mass spectrometry (Py-GC/MS-C-IRMS). The respiration rates and the delta C-13 of the respired CO₂ were measured at regular intervals. After 8 months of incubation, soils incubated with both L. styraciflua and C. canadensis showed a significant change in delta C-13 (delta C-13(final) = -20.2 +/- 0.4 parts per thousand and -19.5 +/- 0.5 parts per thousand, respectively) with respect to the initial value (delta C-13(initial) = -17.7 +/- 0.3 parts per thousand); the same did not hold for soil incubated with P. taeda (delta C-13(final:)-18.1 +/- 0.5 parts per thousand). The percentages of litter-derived C in soil over the total C loss were not statistically different from one litter species to another. This suggests that there is no dependence of the percentage of C input into the soil (over the total C loss) on litter quality and that the fractional loss of leaf litter C is dependent only on the microbial assimilation efficiency. The percentage of litter-derived C in soil was estimated to be 13 +/- 3% of total C loss. [References: 52]