Quantification of PEG-derived CO2 formed in incubations. The mineralization of 13 Clabelled PEGs to 13 CO2 was followed in triplicate incubation bottles. The bottles were placed in an incubation system with automated gas phase 13 CO2 analysis, as previously described. Modifications of the incubation setup (e.g., using pressurized air in the inlet) used in this study are described in section S4 (Supplementary Information).