Contrasting impacts of two subtropical earthworm species on leaf litter carbon sequestration into soil aggregates

Earthworms influence soil carbon sequestration based on the balance between mineralization and stabilization of litter carbon. However, specific impacts and mechanisms of epi-endogeic or endogeic earthworms on litter carbon sequestration need to be clarified. A 126-day laboratory experiment examined how two ecological group earthworms (Amynthas sp. (epi-endogeic species) and Pontoscolex corethrurus (endogeic species)) with distinct feeding habits influence the dynamics and distribution of C-13 labeled Schima superba carbon leaf litter into soil aggregates. Accordingly, three treatments were set as: (i) soil and litter (CK), (ii) soil, litter, and endogeic earthworms (En), (iii) soil, litter, and epi-endogeic earthworms (Ep). Amynthas sp. significantly increased decomposition rate of litter and soil respiration due to the increase of the total phospholipid fatty acids (PLFAs), bacterial PLFAs, and fungal PLFAs and the direct consumption of leaf litter, while P. corethrurus had no effects on these two aspects. Higher proportion of large macroaggregates (> 2000 mu m) was generated in the presence of earthworms, with the highest in the treatment with endogeic earthworms. Besides, both the two species increased the proportion of non-labile organic carbon in large macroaggregates, which was conducive to carbon protection. At the end of incubation, the C-13 signatures of soil indicated that the distribution of litter-derived carbon was significantly affected by the consumption and excretion activities of earthworms, with the result that more carbon was sequestrated in large macroaggregates (> 2000 mu m). Consistent with our hypothesis, Amynthas sp., an epi-endogeic earthworms, could rapidly consume organic residues and stimulate microbial biomass to increase litter decomposition rate and incorporate organic carbon into soil structures, while P. corethrurus, an endogeic earthworms, would stabilize the organic carbon that has been already incorporated into the soil through the formation of large macroaggregates and the transformation of soil organic carbon fractions to more recalcitrant forms.