Acute toxicity tests using earthworms to estimate ecological quality of compost and digestate

Ecological quality of compost and digestate, used as fertilizers for agricultural use, was assessed through an acute ecotoxicological bioassay testing the earthworm Eisenia fetida (Annelida). The test evaluates the earthworm's attitude to dig within 15 min into a soil medium constituted by a mixture of a standardized soil and different concentrations of compost/digestate. According to different classes of behavior responses, the sample is classified as good or bad quality (ON/OFF). The validity of this test was confirmed comparing the observations with the results from a standard chronic test developed by OECD (Organisation for Economic Co-operation and Development). Considering samples from different plants and at different stages of degradations, the replicates, the number of tested concentrations and the measured endpoints (ability to dig, mortality, growth, reproduction), a total of 996 observations were analyzed. The chronic test on earthworms uses the same media as in the acute one and measures the endpoints of mortality and growth within 28 days and reproduction within further 28 days. Thus, the test proposed here can be considered a not expensive and quick laboratory test and its implementation in quality assurance (QA) and quality of product (QC) schemes would enhance the protection of soil fauna. This last purpose is very important considering that, concerning the biological indicators of the quality of compost and digestate, the most important studies focused on the use of plants and microorganisms and scarce attention has been paid to soil invertebrates.