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Sewage sludge applications as soil amendment call for a proper ecological risk assessment due to unexpected delivery of toxic chemicals and materials. Standardized acute toxicity assays have proven to provide limited information in terms of potential hazard for soil organisms. Here, sublethal endpoints as physiological and tissue alterations were proposed as suitable tools for sewage sludge ecological risk assessment. Acute toxicity assay using earthworm Eisenia fetida (OECD n.207) was performed on sewage sludge samples from two wastewater treatment plants (A and B), and mortality, body weight, body morphology, and histology of intestinal cavity were evaluated at the end of the exposure period. Sewage sludge dilutions (1:50 and 1:100) from plant B caused a low percentage of mortality (5%), while none was observed in worms exposed to plant A. However, all specimens showed external body alterations as vesicles, swellings, constrictions, and discoloration. Histological sections highlight damages to epidermis, circular and longitudinal muscles, and chloragogenic tissue regardless of sewage sludge origin. Our findings confirm that standardized end points of acute toxicity are not sensitive enough in worms exposed to sewage sludges, while sublethal endpoints as body morphology and tissue histology are thus more suitable and appropriate endpoints for assessing potential detrimental outcomes and ecological risk for soil biota., Competing Interests: Declarations. Ethical approval: Not applicable. Consent to participate: Not applicable. Consent for publication: All authors agreed with the content, all gave explicit consent to submit, and they obtained consent from the responsible authorities at the institute where the work has been carried out. Conflict of interest: The authors declare no competing interests., (© 2024. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

Soil sorption properties can influence the bioavailability of substances and consequently the toxicity for soil organisms. Current standardised laboratory testing for the exposure assessment of pesticides to soil organisms uses OECD artificial soil that does not reflect the high variation in chemical-physical soil properties found in natural agroecosystems. According to guideline OECD 222, earthworm reproduction tests with Eisenia fetida and the pesticide carbendazim were performed in four natural soils and OECD artificial soil. By using pF 1.6, which ensures a uniformity in actual soil water availability, the control reproduction performance of E. fetida in all natural soils was at the same level as OECD artificial soil. In a principle component analysis, the variation in toxicity between the tested soils was attributable to a combination of two soil properties, namely total organic carbon content (TOC) and pH. The largest difference of 4.9-fold was found between the typical agricultural Luvisol with 1.03% TOC and pH 6.2 (EC10: 0.17 (0.12–0.21) mg a.i. kg−1 sdw, EC50: 0.36 (0.31–0.40) mg a.i. kg−1 sdw) and OECD artificial soil with 4.11% TOC and pH 5.6 (EC10: 0.84 (0.72–0.92) mg a.i. kg−1 sdw, EC50: 1.07 (0.99–1.15) mg a.i. kg−1 sdw). The use of typical agricultural soils in standardised laboratory earthworm testing was successfully established with using the measure pF for soil moisture adjustment. It provides a more application-oriented approach and could serve as a new tool to refine the environmental risk assessment at lower tier testing or in an intermediate tier based approach. [ABSTRACT FROM AUTHOR]