Rapid incorporation of carbon from fresh residues into newly formed stable microaggregates within earthworm casts

Earthworms play an important role in protecting carbon in the soil, but the exact influence of their activity on the distribution and protection of C is still poorly understood. We investigated the effect of earthworms on the formation of stable microaggregates inside newly formed macroaggregates and the distribution of C in them. We crushed (< 250 µm) soil, and subjected it to three treatments: (i) soil + ¹³C‐labelled residue + earthworms (these added after 8 days' incubation), (ii) soil + ¹³C‐labelled residue, and (iii) control (no additions), and then incubated it for 20 days. At the end, we measured the aggregate size distribution, total C and ¹³C, and we isolated microaggregates (53–250 µm) from macroaggregates (> 250 µm) formed. The ¹³C in fine particulate organic matter between and within the microaggregates was determined. Earthworms helped to form large macroaggregates (> 2000 µm). These large macroaggregates contained four times more stable microaggregates than those from samples without earthworms. There was more particulate organic matter within and between microaggregates in macroaggregates in the presence of earthworms. The larger amounts of organic matter inside stable microaggregates in casts than in bulk soil after 12 days of incubation (140 mg ¹³C kg^?1 soil compared with 20 mg ¹³C kg^?1 soil) indicates that these microaggregates are formed rapidly around freshly incorporated residues within casts. In conclusion, earthworms have a direct impact on the formation of stable microaggregates and the incorporation of organic matter inside these microaggregates, and it seems likely that their activity is of great significance for the long‐term stabilization of organic matter in soils.