Wetland is recognized as a large reservoir of organic carbon(C)in terrestrial ecosystems.In this study,the density and variations of organic C in the sediment profiles(totally 16) of the 3 types of wetlands in Sanjiang Plain,Northeast China was investigated.Results demonstrated that the depth of the organic horizon varied in the range of 11～29,18～58 and 70～130 cm;the content of organic C in the horizon in the range of 29～75,124～348 and 187～389g/kg (dry matter);and the density of organic C in the range of 31～48,35～59 and 49～94kg/m³ for marshy meadow,humus marsh,and peat fen,respectively.However,the content and density of organic C in the mineral-illuvial horizon of all the 3 types of wetlands were relatively low and stable.Due to the variations in the content of organic C,a big variation in the density of organic C also occurred not only in the sediment profiles of different types of wetlands but also the same type of the wetland.This variability,together with that in the depth of the organic horizon,resulted mainly in uncer-tainties in the estimates of organic C storage in wetlands.Based on data obtained,it was estimated that the amount of organic C storage per unit area was 8×10³,18×10³ and 72×10³t/km²(with variation coefficients of 40%,49% and 34%) in the organic horizon,and 17×10³,27×10³ and 83×10³t/km²(with variation coefficients of 16%,49% and 30%)in the organic horizon plus 1 m mineral-illuvial horizon of the marshy meadow,humus marsh,and peat fen,respectively.It also suggested that the organic C storage would be greatly underestimated if we only take 1 m as the standard depth in the estimating method for organic C storage.