Safe for the soil? Sublethal costs highlight the ecotoxicity of effluent- versus biosolids-borne contaminant mixtures

Land application of biosolids and effluent is becoming increasingly common, yet the ecotoxicological impacts of complex contaminant mixtures found in these wastewater residuals remain poorly understood. Another key knowledge gap is how the delivery matrix (biosolids vs. effluent) governs these effects. To assess these complex questions, we employed a novel experimental framework using a high-sorption (high organic matter) soil to create a 'best-case scenario' for contaminant immobilization that limited bioaccessibility. This design allowed us to isolate and compare the ecotoxicological impacts of multi-class contaminant mixtures (delivered via aqueous effluent or solid biosolids) on plants, earthworms, and soil microbes across environmentally relevant concentrations. While the soil's sorptive capacity prevented harm to apical endpoints like growth and reproduction, a robust multi-component statistical analysis of sensitive sublethal biomarkers indicated concentration-dependent oxidative stress in both plants and earthworms. Microbial community structure was the most sensitive indicator, for which our data suggest two distinct toxicological mechanisms: highly bioavailable contaminant mixtures in effluent caused notable, albeit variable, reduction in microbial richness, whereas less-bioavailable biosolids-borne contaminants induced selective community restructuring. This work indicates that the contaminant delivery matrix is a critical driver of ecotoxicity. Sublethal harm in this highly sorptive soil suggests that ecological risks from contaminant mixtures in typical, lower organic matter agricultural soils-where bioavailability may be greater-may be underestimated, supporting the need for bioavailability-aware reuse policies.