Total and young organic matter distributions in aggregates of silty cultivated soils

Summary The distribution of organic matter in soil aggregates was investigated by fractionating aggregates and measuring carbon contents. The distribution of recently incorporated organic carbon was analyzed using 13 C natural abundance. The soils of the experiment, which previously only had C 3 vegetation, were cropped to maize, aC 4 plant, for 6 or 23 years. Aggregate size distributions were determined for silty soils with different organic matter contents. Slaking‐resistant macroaggregates were enriched in C as compared to dry‐sieved macroaggregates or to microaggregates, and the C content increased with the size of aggregates. The δ 13 C value was used to calculate the amount of C 3 ‐derived and C 4 ‐derived organic carbon in the fractions. The larger carbon contents in stable macroaggregates were due to young C 4 ‐derived organic carbon (<6 or 23 years), and we concluded that young organic matter was responsible for macroaggregate stability.