Use of stable isotopes (13C) for studying the mobilisation of old soil organic carbon by endogeic earthworms (Lumbricidae)

Endogeic earthworms ingest large amounts of organic matter enclosed in mineral soil. Part of the soil organic matter is mobilised during the gut passage, but the overall effect of earthworms on the dynamics of soil organic carbon (SOC) is poorly understood because the origin and age of the mobilised SOC pool are unknown. To determine whether endogeic earthworms mobilise old SOC pools, we studied the effect of Octolasion tyrtaeum (Savigny) on 13C signatures of CO2 evolved from soil of a maize field incubated in microcosms with and without earthworms for 150 days. Cultivation on this field had changed from wheat (C3 plant) to maize (C4 plant) 23 years ago. Earthworms generally decreased in body mass during the experiment but increased CO2 production. Compared to the initial signature (−22.8 ± 0.1‰PDB), δ13C signatures of O. tyrtaeum were more depleted in the wheat (−23.6 ± 0.3‰PDB) and more enriched in the maize soil (−21.0 ± 0.4‰PDB) at the end of the experiment. The δ13CO2-C signatures in the wheat soil were not affected by earthworms, but earthworms decreased δ13CO2-C values in the maize soil during the second half of the incubation period. This indicates enhanced mineralization of old wheat-derived carbon. The results suggest that lumbricid endogeic earthworms contribute to the mobilisation of old carbon pools in soils.