The nature of the non-mineralized PEG-added C in both sediment and soil incubations was not identified but may have included both residual PEG molecules with comparatively high MW as well as PEG-derived C incorporated into sediment microbial biomass. The latter seems plausible for sediment incubations given that even the non-reacted PEGs underwent extensive mineralization. In this case, the slow and continuous 13 CO2 formation in the sediment during later stages of the incubation would reflect the slow turnover of microbial biomass containing PEG-derived C. While biomass incorporation of PEG-13 C may have also occurred during soil biodegradation, the CNon-Mineralized for the soil incubations were too high to have resulted exclusively from biomass incorporation: maximum carbon use efficiencies of soil microorganisms of around 0.5 have been reported, implying that at most 50 % of the metabolized carbon can become biomass-incorporated. It therefore is likely that a significant fraction of CNon-Mineralized corresponded too PEG molecules with MWs too high to be microbially accessible.