Carbon accumulation in the mineral subsoil of boreal mires

The carbon density and the long-term (apparent) rate of carbon accumulation (LORCA) in the mineral subsoil of boreal mires in Finland formed by paludi fication were studied by comparison to adjacent forest soil sites with simi lar podzolic texture and topography. On the basis of 273 soil profiles from six areas, the mean carbon density in the mineral subsoil of mire sites wa s 1.5-fold higher than in adjacent forest profiles. The average carbon inpu t was 13.6 +/- 1.8 (SE) g m(-2) yr(-1), which differed according to length of time since the onset of paludification. LORCA estimates were highest on the youngest mire sites (< 500 years) and agree with estimates obtained for peat deposits elsewhere in the boreal region. Bogs and topographically fla t areas were found to have higher LORCA values than the fens and the topogr aphically steeper areas. The carbon stored in podzols in boreal forest reac h equilibrium with soil age after 1500-2000 years, while in the mineral sub soil of mires the equilibrium is reached after 4500-5000 pears after initia l paludification. A direct comparison of carbon below mires to the equivale nt depths of peat above indicates that peats are much more efficient in C s equestration. In this study, the average total C density in the mineral sub soil of mire areas to a depth of 70 cm from the bottom of the A horizon wou ld correspond to a peat depth of about 18 cm. Still, the mineral subsoil un der mires is a significant carbon sink, hitherto overlooked, and could acco unt for some 5% of the unaccounted for carbon in the global carbon budget.