It is likely that the low SOM content was not favorable for earthworm development and/or the incorporation of urea had a deleterious impact on them.
For compost and vermicompost treatments, the presence of casts at the soil surface at the end of the first maize cultivation suggests that earthworms were active but died or escaped before plant harvesting.
The absence of earthworms may have been due to either natural fluctuation in earthworm numbers according their lifecycle – although this is unlikely since earthworms are very present in the field at the same time of the year – or by an environmental stress such as temporary high temperatures or low moisture levels.