Secondary salinization in facility agricultural soils detrimentally affects the spatial distribution of earthworm...

The rapid expansion of facility agriculture has triggered extensive secondary soil salinization globally. Earthworms, as the key indicator of soil health, exhibit complex cross-scale interactions with soil salinity that are not yet fully understood. This study selected 34 representative facility greenhouses, along with their adjacent open-field soils and associated earthworm populations. Based on field investigations and multi-model fitting, this study provided quantitative insights into the threshold effects of soil salinity and revealed that soil salt accumulation poses potential ecological risks to earthworm populations. Results showed that greenhouse soils exhibit higher electrical conductivity (810.2 μS/cm vs 275.2 μS/cm) and NO₃-N content (52.2 mg/kg vs 22.0 mg/kg) compared to open-field soils. Moreover, 17.6% of greenhouse sites met the criteria for saline soils (total salt content >6 g/kg). Compared with open-field soil, greenhouse soil exhibited a 50.8% reduction in earthworm density and a 61.6% decrease in biomass. Nitrate exhibited the strongest inhibitory effect on earthworm populations (R² = 0.26, p < 0.01). Structural equation modeling identified soil anion content as the key factor regulating population diversity (path coefficient of -0.695). Additionally, generalized additive model predictions indicated that the alleviation of salt stress would significantly enhance the recovery potential of earthworm populations. This study established the ecological risk threshold of salinity (moderately saline soil, 2-4 g/kg) that triggers earthworm population decline in facility agriculture soils. It also validates the innovative value of integrating multiple models for soil ecological risk assessment, offering a new perspective on evaluating the health of facility agriculture soils.