Ecotoxicological assessment of products resulting from the anaerobic digestion of organic wastes using the collembolan Folsomia candida.

In recent years waste disposal has increasingly become greater as a result of the current economic system based on continued growth in production and consumption of goods and services. It is therefore important to reduce the production and the waste impact on the environment, and to save natural resources by waste retrieving. A way of waste disposal is its treatment by anaerobic digestion, which is a treatment technique of industrial, agro-industrial and urban wastes used for their management and to produce biogas. The digested materials resulting from the process are frequently used in agriculture as fertilizers, soil improvers or adsorbent material but scarce or no evaluations of the biological effects on soil community are available. One of the most important problems of agricultural soils is the alteration of structure, which can be addressed by adding organic complex materials, such as digestate. However, if the added material alters the physico-chemical conditions such as pH or buffer capacity, it may cause ecotoxicological problems, such as a release of metals in soluble form, and it can modify the environmental conditions leading to the fitness decrease of the present populations. So, this practice could exacerbate the decline of biodiversity in agricultural soils rather than improve it. The digestate investigated in this study was obtained from the anaerobic digestion under mesophilic conditions of grape-seeds and kernels of plums, mixed at a 1:1 ratio with a biological sludge from the treatment of urban wastewater. The aim of present research was to evaluate the impact of the digestate added on soil on survival and reproduction of the collembolan Folsomia candida. We exposed F. candida to different concentrations of digestate (0; 2.5%; 5%). The alkaline nature of the digestate leads to the variation of soil pH, therefore we set up twoconcentration test series at two different initial soil pH values (4.5 and 6 respectively). Experimental groups at pH 6 and pH 7, respectively, were also arranged in order to evaluate the biological effects of soil pH alone. The ecotoxicological tests with F. candida were carried out according to ISO protocol (ISO 11267:1999, 2014). One-way ANOVA tests were performed to assess the effect of digestate concentration and soil pH on the biological performances. A Student-Newman-Keuls test was then carried out when appropriate. The results highlighted that the pH values of the experimental soils were dependent on the amount of the added digestate and the initial pH of the artificial soil. A statistically significant effect on survival was associated with the increase of the pH of the experimental soil. No effect on survival was directly caused by the addition of digestate to the experimental ground, as the springtail survival significantly decreased with the higher digestate concentration used (5%) only in the experimental group at higher soil pH. No effect on reproduction was detected related to the addition of digestate to the experimental ground, while a statistically significant effect on reproduction was associated with the increase of the pH of the experimental soil. In fact, reproduction resulted significantly modified depending on pH reached in soil. In soils where the higher digestate concentration was used (determining a higher pH value: pH=8) the juvenile number strongly decreased with respect to other experimental groups. In all experimental groups characterized by pH ≥ 7 their reproduction was negatively affected independently from the digestate added (0; 2.5%; 5%). In the group with 2.5% of digestate and experimental soil pH = 6 the number of juveniles was not different from the number of juveniles found in the control group with experimental soil pH = 6. The effect on the survival and reproduction of the springtail F. candida, who normally prefer to settle on weakly acid soils, seemed related to the alkaline nature of the digestate, rather than to its other characteristics. This study recommends an appropriate evaluation in advance of the physical chemical characteristics of agricultural soils and the digestate before proceeding to its spreading in the environment.