TRACKING THE FATE AND RECYCLING OF 13C-LABELED GLUCOSE IN SOIL

A short-term incubation of soil amended with 13C-glucose was conducted to determine the extent of labeled C recycling that might occur within the microbial community. Changes in the production and isotopic composition of CO2 and biomass suggest that two phases of microbial activity occurred after the glucose addition. The initial phase due directly to the metabolism of the added glucose was characterized by an increase in biomass and a high growth efficiency. A second phase appeared to be driven by less available substrates (e.g., cell wall structures, soil organic matter) and characterized by insignificant changes in biomass but significant generation of CO2 suggestive of low growth efficiency. Glucose-C supported 12 to 73% of the CO2-C evolved and 17 to 21% of biomass-C, suggesting glucose was the principle energy rather than a C source during the 15- to 48-hour phase of the incubation. Variation in δ13C composition of individual phospholipid fatty acids (PLFA) during the incubation indicated that different components of the microbial community played different roles in the cycling of the added glucose. The most enriched δ13C values were initially those PLFA associated with Gram-positive bacteria, suggesting they were responsible for much of the initial incorporation. By contrast, at the end of the 48-hour incubation, 4 of 24 PLFA biomarkers were not labeled with 13C. Actinomycetes, however, probably played a larger role in the use of recycled glucose-derived C, as suggested by the enrichment in 13C of 10-methyl 18:0 PLFA after the exhaustion of glucose. Results from this study show that the element of time needs to be considered carefully in the interpretation of any stable isotope labeling and biomarker study.