Branched Glycerol Dialkyl Glycerol Tetraethers (GDGTs) are membrane spanning lipids synthesised by as yet unknown bacteria that thrive in soils and peat. In order to obtain more information on their ecological niche, the stable carbon isotopic composition of branched GDGT-derived alkanes, obtained upon ether bond cleavage, has been determined in a peat and various soils, i.e. forest, grassland and cropland, covered by various vegetation types, i.e., C 3 - vs. C 4 -plant type. These δ 13 C values are compared with those of bulk organic matter and higher plant derived n -alkanes from the same soils. With average δ 13 C values of −28‰, branched GDGTs in C 3 soils are only slightly depleted (ca. 1‰) relative to bulk organic carbon and on average 8.5‰ enriched relative to plant wax-derived long-chain n -alkanes ( n C 29 – n C 33 ). In an Australian soil dominantly covered with C 4 type vegetation, the branched GDGTs have a δ 13 C value of −18‰, clearly higher than observed in soils with C 3 type vegetation. As with C 3 vegetated soils, branched GDGT δ 13 C values are slightly depleted (1‰) relative to bulk organic carbon and enriched (ca. 5‰) relative to n -alkanes in this soil. The δ 13 C values of branched GDGT lipids being similar to bulk organic carbon and their co-variation with those of bulk organic carbon and plant waxes, suggest a heterotrophic life style and assimilation of relatively heavy and likely labile substrates for the as yet unknown soil bacteria that synthesise the branched GDGT lipids. However, a chemoautotrophic lifestyle, i.e. consuming respired CO 2 , could not be fully excluded based on these data alone. Based on a natural labelling experiment of a C 3 /C 4 crop change introduced on one of the soils 23 years before sampling and based on a free-air CO 2 enrichment experiment with labelled CO 2 on another soil, a turnover time of ca. 18 years has been estimated for branched GDGTs in these arable soils.