Field-scale labelling and activity quantification of methane-oxidizing bacteria in a landfill-cover soil

Aerobic methane-oxidizing bacteria (MOB) play an important role in soils, mitigating emissions of the greenhouse gas methane (CH4) to the atmosphere. Here, we combined stable isotope probing on MOB-specific phospholipid fatty acids (PLFA-SIP) with field-based gas push-pull tests (GPPTs). This novel approach (SIP-GPPT) was tested in a landfill-cover soil at four locations with different MOB activity. Potential oxidation rates derived from regular- and SIP-GPPTs agreed well and ranged from 0.2 to 52.8 mmol CH4 (L soil air)-1 day-1. PLFA profiles of soil extracts mainly contained C14 to C18 fatty acids (FAs), with a dominance of C16 FAs. Uptake of 13C into MOB biomass during SIP-GPPTs was clearly indicated by increased d13C values (up to c. 1500 parts per thousand) of MOB-characteristic FAs. In addition, 13C incorporation increased with CH4 oxidation rates. In general, FAs C14:0, C16:1?8, C16:1?7 and C16:1?6 (type I MOB) showed highest 13C incorporation, while substantial 13C incorporation into FAs C18:1?8 and C18:1?7 (type II MOB) was only observed at high-activity locations. Our findings demonstrate the applicability of the SIP-GPPT approach for in situ quantification of potential CH4 oxidation rates and simultaneous labelling of active MOB, suggesting a dominance of type I MOB over type II MOB in the CH4-oxidizing community in this landfill-cover soil.