Sewage sludge application strongly modifies earthworm impact on microbial and biochemical attributes in a semi-arid calcareous soil from Iran

• Earthworms decreased soil microbial biomass content and turnover time.
• Microbial activity and fungal respiration were increased by earthworms.
• Earthworm effects on microbial indicators were decreased with sludge addition.
• Modification of earthworm effects was greater with addition of endogeic species.
• Interaction effect between earthworm species on microbial performance was additive.

Application of municipal sewage sludge (MSS) may modify the influence of earthworm activity on soil microbial and biochemical properties through an increase of substrate availability. However, the effect of earthworm activity on microbial community is poorly known in calcareous soils amended with MSS. Such knowledge would lead to better understanding of how earthworms and microorganisms interact following MSS application in agroecosystems. This study aimed to investigate how MSS application can modify the impact of earthworms on microbial performance and biochemical attributes of a calcareous soil amended with MSS. Experimental treatments were MSS application (without and with 1.5% MSS) and earthworm addition (no earthworm, epigeic Eisenia fetida, endogeic Aporrectodea caliginosa and a combination of the two earthworm species) setup as a 2 × 4 full factorial experiment using a completely randomized design with three replications for each treatment combination under greenhouse conditions over 90 days. Addition of MSS had a positive effect on the measured biochemical and microbiological indicators of the study soil. Municipal sewage sludge application resulted in a significant (p < 0.001) increase in the amounts of microbial biomass carbon (MBC) and basal respiration rate (RR); but without an effect on the level of dissolved organic carbon (DOC). While the presence of earthworms decreased the quantity of DOC and MBC (p < 0.001); other soil attributes such as basal respiration rate, the metabolic quotient (qCO2) and the ratio of fungal to bacterial (F/B) respiration were all increased significantly (p < 0.001) with earthworm addition. Nevertheless, the magnitude of the earthworm effect was largely dependent upon the MSS treatment. Application of MSS decreased the stimulatory effect of earthworms on soil microbial indicators and this effect was less pronounced in the presence of E. fetida than A. caliginosa species. This is probably due to the preference of epigeic E. fetida to feed on MSS as an essential food resource rather than only feeding and grazing on microbial populations. In contrast, the endogeic A. caliginosa species was not dependent upon MSS as food resources. Moreover, the interaction effect between the two earthworm species on soil microbial and biochemical properties was mostly additive in nature (without interaction) in both MSS-amended and unamended soils. Results indicated that earthworm effect and its interaction with MSS were important factors for soil microbial biomass, community composition and performance and that earthworm effects should decrease in MSS-treated soils.