Evaluation of two different standard soils for their use in ecotoxicological tests

The quality of soil as a resource is evaluated with ecotoxicological tests, using soil organisms such as Anellida, Nematoda and Collembola. The International Organization for Standardization (ISO) has developed ecotoxicological tests to obtain information on biological effects of contaminants in soil and to improve the data obtained with the conventional chemical analysis. The ISO guideline 11267 (1999, 2014) describes a standardized method that is based on the determination of sublethal effects of contaminated soils on the collembolan Folsomia candida. However, the standard artificial soil OECD (1984), firstly proposed in ISO 11267 (1999), does not represent the heterogeneity of natural soils, possibly invalidating the transfer of laboratory evaluations to the natural environment. As a matter of fact, the second edition of the ISO 11267 (2014) involves the use of standard natural soils in addition to the artificial soil OECD. The aim of this work is to compare by soil ecotoxicological tests two standard soils included in ISO 2014: the artificial soil OECD and the natural soil LUFA 2.2. The comparison of the two soils was performed assessing the effect on survival and reproduction of F. candida of a secondary product, the digestate, which is used in agricultural practices and it is derived from the anaerobic digestion of agricultural crops and manure. The digestate (pH 7.3) was tested at two concentrations (2%, 4%) in the two soils; two control series without digestate were also prepared. The digestate added did not affect the survival of F. candida in both tested soils. However, the digestate added to the LUFA soil negatively affected the reproduction of F. candida: in the experimental series with 2% digestate, the number of juveniles dramatically decreased with respect to control group, and no juveniles were found in LUFA soil treated with 4% digestate. In the experimental series with OECD soil no effect on reproduction of F. candida was detected. Many Authors affirm that F. candida reproduction is negatively affected by soil pH values >6.6. In our experiment, the pH value of the OECD soil treated with digestate did not change compared to the initial pH value=6, while the pH values of the treated LUFA soil reached 6.8 and 7.4 with 2% and 4% digestate, respectively. The negative effect on reproduction could therefore be attributed to the increase of pH values. Thus, our study deeply questions the interpretation of the results of standardized toxicity tests.