Soil-specific ecological risk thresholds for tetracycline derived from multi-trophic toxicity data and species sensitivity distribution.

Tetracycline (TC) is a veterinary antibiotic that can enter agricultural soils through manure application, raising concerns about its ecological risks in terrestrial ecosystems. Soil-specific ecological risk thresholds remain poorly defined because terrestrial toxicity data are limited, sometimes necessitating reliance on aquatic data or aquatic-to-soil extrapolation. Soil toxicity assays were conducted using Chinese flowering cabbage (Brassica parachinensis L.), two cultivars of pak choi (B. chinensis L.; T668 and 8933), lettuce (Lactuca sativa L.), the earthworms Eisenia fetida and E. andrei, the springtails Folsomia candida and Protaphorura bicampata, and the nematode Caenorhabditis elegans. TC generally inhibited plant growth and soil invertebrate survival or reproduction in a concentration-dependent manner, with clear species- and endpoint-dependent differences. Based on nominal TC concentrations, 10% effect concentration (EC10) values ranged from 1.11 to 224.73 mg/kg for plants and from 6.78 to 127.48 mg/kg for soil invertebrates. Representative EC10 values were selected according to endpoint sensitivity and consistency within organism groups for species sensitivity distribution (SSD) analysis. The normal distribution provided the best acceptable fit (P > 0.05; root mean square error = 0.060). The fitted SSD yielded a 5th percentile hazardous concentration (HC5) of 1.646 mg/kg (95% confidence interval: 0.44-6.11 mg/kg). Applying an assessment factor of 5 gave a predicted no-effect concentration for soil (PNECsoil) of 0.3292 mg/kg. To our knowledge, no soil quality standard for TC has yet been established in China. These results provide soil-specific references for TC ecological risk assessment and reduce reliance on aquatic-to-soil extrapolation.