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Summary What processes control the accumulation and storage of carbon (C) in the mineral subsoil beneath peat? To find out we investigated four podzolic mineral subsoil profiles from forest and beneath peat in Lakkasuo mire in central boreal Finland. The amount of C in the mineral subsoil ranged from 3.9 to 8.1 kg m-2 over a thickness of 70 cm and that in the organic horizons ranged from 1.8 to 144 kg m-2 . Rates of increase of subsoil C were initially large (14 g m-2 year-1 ) as the upland forest soil was paludified, but decreased to < 2 g m-2 year-1 from 150 to 3000 years. The subsoils retained extractable aluminium (Al) but lost iron (Fe) as the surrounding forest podzols were paludified beneath the peat. A stepwise, ordinary least-squares regression indicated a strong relation (R 2 = 0.91) between organic C concentration of 26 podzolic subsoil samples and dithionite–citrate–bicarbonate-extractable Fe (negative), ammonium oxalate-extractable Al (positive) and null-point concentration of dissolved organic C (DOCnp ) (positive). We examined the ability of the subsoil samples to sorb dissolved organic C from a solution derived from peat. Null-point concentration of dissolved C (DOCnp ) ranged from 35 to 83 mg l-1 , and generally decreased from the upper to the lower parts of the profiles (average E, B and C horizon DOCnp concentrations of 64, 47 and 42 mg l-1 ). The DOCnp was positively correlated with percentage of soil C and silt and clay content. The concentration of dissolved organic C in pore water in the peat ranged from 12 to 60 mg l-1 (average 33 mg l-1 ), suggesting that the sorptive capacity of the subsoil horizons for C had been exhausted. We suggest that the increase of C contents in the subsoil beneath mires is related to adsorption of dissolved organic C and slow mineralization under anaerobic conditions. [ABSTRACT FROM AUTHOR]