Effects of Organic and Synthetic Fertilizers on Earthworm Survival and Growth: Implications for Vermicomposting Feedstock Contamination

The compatibility of fertilizer inputs with vermicomposting systems remains a critical factor influencing compost quality and biological efficiency. This study evaluated the short-term effects of five synthetic fertilizers (urea, ammonium sulfate, NPK, diammonium phosphate, and a phosphorus-based fertilizer) and two organic amendments (animal dung and poultry waste) on the survival, growth, and performance of Eisenia fetida under controlled conditions. Treatments were applied at concentrations ranging from 5 to 35 g per 5 kg soil, alongside an untreated control, and assessed over a 96-h exposure period. Significant differences in earthworm responses were observed among fertilizer types (p < 0.05). Organic amendments, particularly poultry waste, supported 100% survival and resulted in substantial biomass gains (up to 55%), indicating high substrate compatibility. In contrast, nitrogen-rich synthetic fertilizers (urea and ammonium sulfate) induced acute toxicity, causing up to 100% mortality within 48 h across all tested concentrations. Compound fertilizers (NPK and phosphorus-based inputs) exhibited intermediate, dose-dependent effects, with limited tolerance at lower concentrations. Regression and correlation analyses revealed a positive association between available phosphorus and biomass gain (p < 0.05), whereas potassium showed a negative relationship with survival. Overall, organic substrates demonstrated superior suitability for vermicomposting, while direct incorporation of synthetic fertilizers—particularly nitrogen-based formulations—adversely affected earthworm performance and system stability. These findings highlight the ecotoxicological risk posed by synthetic fertilizer contamination of organic feedstocks and provide practical guidance for feedstock screening and contamination control prior to vermicomposting.