Size–density fractionation in combination with the use of 13C analysis yielded detailed information on soil organic matter (SOM) dynamics and stabilisation in aBrachiaria humidicola pasture (C4), established after rain-forest (C3) by slash and burn 16 year prior to soil sampling. The mediumterm13C marker was supplemented with ashort-term14C marker by incubating the soil over 75 days with homogeneously-labelled Lolium perenne. The use of the medium-term marker (13C) showed a potential for density fractionation that would not have become apparent using the short-term marker (14C) on its own. To a large extent this disparity has to do with limiting characteristics of the used 14C-labelled material (small particle size and high content of water soluble carbon). The 14C-labelled material was immediately present in all fractions but mainly corroborated the results from the 13C determination. Both 14C and 13C measurements indicated the transfer of carbon from large light fractions to fine heavy organomineral fractions. The ‘youngest’ C4 fractions floated on water, whereas the ‘intermediately aged’ C4/C3 fractions were intimately associated with the mineral phase through biological processes, and came out as heavy. The ‘oldest’ C3 fraction consisted of stabilised free organic material that had not attained high mineral affinity, and therefore had intermediate density. The changes in 14C specific activity of the isolated fractions in the shortterm indicated that these fractions were not homogeneous. This suggests that there will be limited scope for tying the size–density fractions and the conceptual homogenous pools used in SOM models together without further characterisation of these pools or by using alternative decay models.
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