Dose-dependent effects of urea fertilizers on the green toad (Bufotes sitibundus): From growth enhancement to lethal toxicity

Abstract

This study elucidates urea fertilizers' dual, concentration-dependent effects on the green toad (Bufotes sitibundus, Pallas, 1771), revealing a nonlinear dose–response pattern that may reflect a potential hormetic effect at low doses versus acute toxicity at elevated concentrations. Larvae exposed to urea gradients (5–50 mg/L and 5–7 g/L) were evaluated for survival, metamorphic dynamics, and growth indices. At 10 mg/L, larvae exhibited optimal outcomes: 95 % survival (57/60 metamorphosed), accelerated metamorphosis (50 days), and superior morphometrics (mean length: 30.72 mm; post-metamorphic weight: 0.56 g), possibly consistent with hormetic stimulation. Conversely, high concentrations (50 mg/L) induced 71 % mortality (44 deaths), stunted growth (28 mm; 0.41 g), and oxidative stress, while 5–7 g/L caused complete embryonic lethality within 6 days. Intermediate doses (15–20 mg/L) showed reduced mortality (78.3 %–65 % survival) but delayed development, reflecting resource-stress trade-offs. Statistical analyses (Kaplan-Meier, ANOVA, Tukey HSD) confirmed significant dose-response disparities (p < 0.05). These findings underscore the critical balance between agricultural productivity and ecological conservation, advocating for stringent regulation of fertilizer use in amphibian habitats to mitigate biodiversity loss. The study highlights the necessity of context-specific thresholds in ecotoxicological risk assessments, emphasizing that sublethal doses may mask long-term ecological impacts through hormesis-driven adaptation.