- Fate of contaminants. For mineral or organic contaminants, we studied the mechanisms and/or the processes of degradation or trapping by the soil constituents (de Santiago-Martín et al., 2015; Spadini et al., 2018). Total contents were measured either to spatialize the contamination (in the field, following land management or anthropic pressure) or to track changes with time after some inputs (in field or ex-situ in microcosms), and linked with effects to specific soil organisms (Nélieu et al., 2016). We were also interested in the management of these contaminants, and thus looking for avoiding their dispersal thanks to the substitution of one product by another (EDIFIS project) or by favoring their biodegradation or limiting their bioavailability (GESIPOL or PIEGEACHLOR projects) or during the phase of waste treatment (Ardo et al., 2015; TRICLOSAN project). In all cases, these studies require adapted analytical methods: analytical developments thus constitute an essential part of the researches (Goulas et a.l, 2017). One project (CICESOL project) was specifically dedicated to the exploitation of the 90 years old long-term bare fallow ’42-plots’ experiment of INRA-Versailles and its historical soil archive laid on a Luvisol, representative of large areas of north-western Europe. Parcels without or with continuous application of a series of N, P, and K-fertilizers and basic and organic amendments were studied to assess changes in their biogeochemical and physicochemical properties. In the reference soils without fertilizers, cumulative inputs of atmospheric deposition of metal contaminants (sum of Pb+Zn+Cr+Co) could be budgeted and reached 200 kg/ha in 85 years. Under fertilization, we assessed that some soil processes can be activated like acidification and mineral dissolution as well as lixiviation of trace metals (Cd, Ni, Co, Mn) under ammonium fertilizers, or clay leaching and mobility of associated trace elements (Tl, Sc, Ni, Cr, As) in sodium and/or potassium based fertilizer.
Biological contaminants, such as cyanotoxins (microcystin-LR), have also been considered in the context of soil irrigation with contaminated water, transfer to crops, and ecotoxicological impact (Corbel et al., 2014; 2015). It has been shown that a high risk of toxin leaching from the soil toward groundwater occurred. Exposure in soil to realistic environmental microcystins concentrations affected seed germination, depending plant species. It was also highlighted disturbances in soil bacteria functioning through soil nitrification process.
- Relationship between speciation of the contaminants and the effects on soil organisms. We specifically studied this relationship using the concept of bioavailability as described in the ISO 17402 norm, while focusing on the role of the organic matter as both ligand for contaminants and food source for organisms.
A conceptual model of causal relationship was elaborated, and validation was looked for with data produced from microcosms studies with earthworms exposed to a gradient of in situ contamination. Causal relationship between availability of contaminants in the soil and uptake of contaminants by earthworms (called environmental bioavailability) was found dependent of the type of contaminant (Beaumelle et al., 2015) (Fig. 19). Subcellular distribution of Cd in earthworm was found dependent of the total content of soil Cd, while subcellular distribution of Pb was only related to the total internal Pb content in the worm. No causal relationship could be found between soil metal availability and the glycogen contents in earthworms as indicator of toxicological bioavailability, but rather we found that soil texture and extractable metals with CaCl2 were the only two parameters explaining the internal lipid and protein earthworm contents (Beaumelle et al., 2014). Subcellular fractions of metals in earthworms were found better biomarkers than the corresponding total internal contents (Beaumelle et al., 2017). Finally, a structural equation model was used to confirm that a causal relationship can be established when dealing with Cd and Pb but not with Zn, and to assess whether some indicators are robust or not (Beaumelle et al., 2016).
- Bioavailability and mixture of contaminants. A main work was done through an ANR project (CEMABS project) with the conjunction of Soil and Ecotox teams of ECOSYS, where the importance of taking into account the bioavailability of antibiotic compounds was assessed when dealing with the effects on the soil microbial functioning. The context of this study was the recycling of urban or agricultural organic wastes. Microbial activities and PICT (Pollution Inducing Community Tolerance) were used as powerful tools assessing the relation between the nature of the organic inputs and the related fluxes of contaminants, and the effects on the nutrient recycling soil functions (Goulas et al., 2014; David et al., 2016; Crouzet et al., 2015, 2016.). Furthermore, the works initiated on the effects of the mixture of contaminants, either mixtures of pesticides (IMPEC project) or organo-mineral mixtures (antibiotic – metal in the CEMABS project) allowed identifying microbial activity descriptors like nitrification as particularly sensitive for microbial communities and their functions. Coupling bioassay-based experiments to toxicological contaminant interaction models (i.e. concentration attention) allowed us to assess the genericity of the mixture models for the environmental risk assessment of contaminant effects on soil microbial functions (Chatillon et al., 2017). This mixture approach allows to identify contaminant interaction, such as synergistic or antagonism effects, but concerning mainly short-term effects on potential microbial activities when using bioassay based conditions.