The 15N and 13C values of the earthworms in the 10th month were lower compared with that in the 6th month (Fig. 4). Ponsard and Arditi [32] considered that seasonal differences in isotopic ratios of animal samples were not large.Accordingly, we speculated that the decrease in 15N and 13C of the earthworms from 6th month to 10th month was likely resulted from the accumulation of earthworm feeding sewage sludge. Average weight of the earthworm samples in the 10th month was 2.04–2.43 times that in the 6th-month samples (Fig. S1 of the SD), showing that the weight of the earthworm individuals rose greatly, implying that more organic matter from sewage sludge with low 15N and 13C values was assimilated to synthesis the earthworm body, and thus 15N and 13C natural abundances of the earthworm body decreased further.