To elucidate the mechanisms responsible for the long-term retention of soil C, soil samples were retrieved from a paleosolic A horizon (M-bur) buried underneath a Bronze Age Mound and from waterlogged anaerobic (M-an) and aerobic zones (M-ae) within the mound. For comparison with a modern soil, samples were taken from the Surrounding arable field (Ar). The soils were characterized by cross-polarization magic angle spinning C-13 nuclear magnetic resonance spectroscopy, by phospholipid fatty acid (PLFA) analysis, and by CO2 production during incubation. At sampling, the Ar, M-an, M-ae, and M-bur soils contained 22.6, 17.9, 6.5, and 6.2 g C kg(-1) soil, respectively. Compared with the Ar and M-an soils, the M-bur and M-ae soils were depleted in alkyl C, N-alkyl C, and O-alkyl C and enriched in aromatic and carboxylic groups. The content of PLFA in the mound soils was much lower than normally found for arable soils and the ability of the deprived biomass to decompose glucose was retarded and included a 2- to 3-wk lag phase. The lability of C left under aerobic conditions for more than three millennia did not differ significantly from that of C in the contemporary Ar soil. Although the chemical nature Of the C in the Ar and M-an soils was similar, C that had resided under anaerobic conditions was less labile than C from the arable soil. We consider the persistence of C in the unwaterlogged parts of the burial mound to result from physical restrictions imposed on gas exchange and water availability, and thus restricted microbial activity, rather than from biochemical recalcitrance of the remaining C.