Assessment of chronic diesel toxicity in soil using multiple endpoints in Eisenia fetida

Abstract

Diesel-derived hydrocarbons persist in soil matrices and can exert prolonged pressure on invertebrates even when overt toxicity is limited. Here, chronic responses of Eisenia fetida were examined following 28-day exposure to artificial soil amended with diesel fuel (0.4, 1.0, and 1.6 mL g⁻¹). Effects were evaluated across survival, biomass change, oxidative stress biochemistry, and clitellum histology to capture responses spanning organismal, biochemical, and tissue scales. Mortality increased with both concentration and time, yet cumulative mortality did not separate statistically among treatments because of pronounced variability within exposure groups. Changes in body mass were similarly inconsistent, with the greatest dispersion occurring at intermediate diesel levels, indicating that growth was not a monotonic function of exposure intensity. Activities of antioxidant enzymes (superoxide dismutase, catalase, glutathione peroxidase), levels of malondialdehyde, and total protein content remained comparatively stable throughout the exposure period, providing little evidence for sustained oxidative imbalance under these chronic, sublethal conditions. In contrast, clitellum histology revealed progressive and concentration-dependent injury, including epithelial degradation, glandular atrophy, disruption of muscle organization, and focal necrosis. The divergence between weak or variable responses at the survival and biochemical levels and consistent tissue-level damage indicates that conventional endpoints underestimated chronic diesel toxicity. These findings emphasize the diagnostic value of histopathology for identifying environmentally relevant effects of petroleum hydrocarbons in soil systems, particularly where biochemical markers and survival fail to reflect ongoing structural injury.