Four soils with 6, 12, 23 and 46% clay were fractionated according to particle size after incubation for 5–6 years with 14C labelled straw, hemicellulose or glucose: 6–23% of the 14C was still present and the amount increased with increasing content of fine particles.

clay fractions contained 66–84% of the 14C and the silt fractions accounted for 4–19%. <2% was found in the sand fractions and 4–9% was water soluble. The distribution of the native C was: clay, 46–68%; silt, 20–31%; sand, 2–7%.

The clay fractions had higher relative proportions of 14C than of native C, the reverse being true for the silt fractions. This distribution pattern was not directly related to soil clay content or to kind of organic amendment.

The C enrichment factor of clay and silt fractions (per cent C in fraction/per cent C in whole soil) increased with decreasing fraction size for both native and 14C. However, clay enrichment factors were higher for 14C than for native C, whereas silt enrichment factors were lower.

A soil (9% clay) that had been incubated in the field for 18 years with 14C labelled straw was also analysed. Labelled C content at sampling was 9% of the initial value. In contrast to the other soils the distribution of labelled and native C was similar in the clay and silt fractions, which contained 55% and 33% of the whole soil C, respectively.

The results indicate that clay-bound organic matter may be important in mediumterm organic matter turnover, whereas silt-bound organic matter may participate in longer-term organic matter cycling.