We present a method for determining the natural abundance radiocarbon ((14)C) content of soil microbial biomass (SMB) based on existing fumigation extraction procedures. We applied the technique to soils from the foreland of the Odenwinkelkees glacier in the Austrian Alps, which has a well-characterised chronosequence of soils at different stages of development. Across the chronosequence, SMB contained post-bomb levels of (14)C, suggesting it was substantially composed of carbon that had been fixed since the 1960s. Comparison of our results with previous findings from the same site showed that at most stages in the sequence the SMB had a similar (14)C content to the bulk soil organic matter (SOM). However, soil respired CO(2) was (14)C-depleted relative to SMB, indicating that at least a component of the microbial community was mineralising some older carbon. In the most recently exposed soils, SMB was enriched compared compared to both soil respiration and SOM, suggesting that a small component of the microbial biomass that utilises older carbon contributes disproportionately more to the CO(2) efflux. Although other interpretations are possible, this explanation is consistent with the notion that early on in the succession a large proportion of the microbial biomass is dormant. (C) 2011 Elsevier Ltd. All rights reserved. ; Garnett, Mark H. Bol, Roland Bardgett, Richard D. Wanek, Wolfgang Baeumler, Rupert Richter, Andreas