Stabilization of soil organic matter by earthworms is connected with physical protection rather than with chemical changes of organic matter

To explain underlying mechanisms of a lower C loss in the earthworm vs. mechanically mixed treatment, we separated SOM fractions in order to gain pools defined in the Rothamsted model. Chemical differences between initial litter and the active and slow pool of SOM obtained by fractionation were studied. No significant differences between the earthworm and mechanically mixed treatment were found in C, nitrogen (N), and phenol contents, composition of major chemical groups of litter studied by solid-state 13C NMR spectroscopy, and composition of aromatic components of SOM studied by analytical pyrolysis (Py GC/MS). This lack of differences in chemical composition suggests that greater SOM sequestration in the earthworm treatment is likely to be connected with physical protection of SOM inside cast aggregates rather than with chemical changes in SOM mediated by earthworms.