Assessing the chronic toxicity of spreading organic amendments on agricultural soil: Tests on earthworms and plants

Recycling organic wastes on agricultural soils improves the soil quality, but the environmental and health impact of these organic amendments closely depends on their origins, their bio-physicochemical characteristics and the considered organisms potentially affected. The aim of this study was to assess the potential chronic ecotoxicity of spreading organic amendments on agricultural soils. To do this, we characterized three different organic amendments: sewage sludge from an urban wastewater treatment plant, cow manure and liquid dairy manure. Their chronic ecotoxicity was studied through assays exposing earthworms of the species Eisenia fetida and two plants: Medicago sativa and Sinapis alba. Of the three amendments, the sewage sludge presented the highest concentrations of micropollutants and a considerable fraction of available and biodegradable organic matter. The cow manure and liquid dairy manure had lower chemical contamination and similar characteristics with lower biodegradable fractions of their organic matter. No chronic phytotoxicity was evidenced: on the contrary, particularly with sewage sludge, the germination rate and aerial and root biomass of the two plants increased. Considering earthworms, their biomass increased considerably during the reproduction assays in soil amended with sewage sludge, which contained the more bioavailable organic matter. Nonetheless, the earthworms presented an inhibition close to 78% of the production of juveniles when exposed to sewage sludge exceeding 20 g.kg-1 DW (that means 2 times the agronomic dose). This reprotoxic effect was also observed in the presence of liquid dairy manure, but not with cow manure. At the end of the assays, the glycogen and protein reserves in earthworms exposed to sewage sludge were inferior to that of control earthworms, respectively around 50% and 30%. For the earthworms exposed to liquid dairy manure, protein and lipid reserves increased. In the case of liquid dairy manure, this reprotoxic effect did not appear to be linked to the presence of micropollutants. In conclusion, our results confirm the need to use several ecotoxicity assays at different biological levels and with different biological models to assess the ecotoxic impacts of soil amendments. Indeed, although certain organic wastes present a strong nutritional potential for both plants and earthworms, a not inconsiderable risk was apparent for the reproduction of the latter. An integrated ecotoxicity criterion that takes into account a weighted sum of the different results would guide the utilization of organic amendments while ensuring the good health of agricultural ecosystems.