Ecotoxicological assessment of waste-derived organic fertilizers and long-term monitoring of fertilized soils using a multi-matrix and multi-species approach

The present study investigates the ecotoxicity of 7 biofertilizers, including biowaste-derived organic matrices. Real-field tests were conducted to assess the impacts of soil fertilization with sewage sludge digestate from high-solid thermophilic anaerobic digestion (HSTAD) compared to those obtained on non-amended and urea-fertilized soils. The physical-chemical and ecotoxic impact of HSTAD digestate on soil was monitored for 12 months, at 5 time points and 2 soil depths, on a maize field divided in 3 portions (non-treated, fertilized with urea, amended with digestate). The chemical and physical characteristics of the soil were previously analyzed for 3 years to provide a long-term outlook of the impacts of biofertilizer application. Seven bioindicators were utilized for direct (on whole soil) and indirect (on soil elutriates) ecotoxicological tests on fertilizers and amended soils, including plant seeds (Lepidium sativum, Sorghum saccharatum, and Sinapsis alba), the aquatic organism Daphnia magna, the alga Raphidocelis subcapitata, the luminescent bacterium Aliivibrio fischeri, and the Nematode Caenorhabditis elegans. No serious negative effects on soil fertilized with HSTAD digestate were evidenced. Conversely, bioassays rather showed positive effects, encouraging the utilization of HSTAD digestate in agriculture, considering the proper concentrations of use. The obtained data were interpolated and a test battery integrated index was generated, confirming the absence of ecotoxicological risk for the soils amended with the applied fertilizers. The long-term evolution of the physical-chemical soil characteristics (including the concentrations of potential contaminants) was similar for both HSTAD digestate and urea application as well as for non-fertilized soil, indicating no negative effects due to digestate application on land. On the contrary, digestate application improved the content of stabilized organic matter and nutrients in soil. This study proposes a more correct approach to ecotoxicity assessment of fertilized soils for biofertilizer evaluation and demonstrates the long-term safe application of HSTAD digestate on agricultural soil.