Incubation time was controlled based on the development of δ13C values of CO2 in the headspace. δ13C of CO2 was measured by Thermo Finnigan Trace GC coupled to a Thermo Finnigan Delta plus XP isotope ratio mass spectrometer (IRMS). Deviations of triplicate isotopic measurement of δ13C of CO2 were between ± 1‰ and ± 1,000‰ (for CO2 with label uptake of >10,000‰). Slurries were harvested at five time points for detailed analyses.