Dissolved organic matter (DOM) is often suspected to play a major role in the mobility of organic pollutants in soils. However, the impacts of land-use on such processes still remain poorly understood. We aimed to quantify the sorption of organic pollutants to subsoils and the possible effects of DOM. Two contrasted soil types and soil management systems were chosen: a cambisol with leys introduction in a maize-wheat-barley rotation and a luvisol amended with different organic waste composts in a maize-wheat rotation.
DOM samples were extracted from the topsoil horizons (0-28 cm) for different treatments at each system. Low concentration ranges of dissolved organic carbon (DOC) were used in reference to field leachate data. Labeled 14C molecules, three pesticides: isoproturon (N-N-dimethyl-N’-[4-(1-methylethyl) phenyl] urea); dicamba (3,6-dichloro-2-methoxy benzoic acid); diflufenican (N-(2,4-difluorophenyl)-2-[3-(trifluoromethyl)phenoxy]-3-pyridinecarboxamide) and an organic waste contaminant, n-dibutylphthalate were studied. Batch equilibration experiments were performed to quantify the sorption of these chemicals to subsoil horizons with or without addition of DOM.
For both systems, sorption was higher for diflufenican followed by isoproturon and dicamba suggesting a stronger affinity of hydrophobic compounds for the subsoil. Pesticide sorption decreased with depth following the decrease of the organic content in the profile, confirming the strong affinity for soil organic matter. Sorption was higher in the luvisol profile relating to its physico-chemical properties. Contrastingly to pesticides, n-dibutylphthalate showed an increased sorption with depth and the cambisol profile. This suggested that others interactions were involved during its sorption. No significant differences were observed with the different DOM tested. This was explained by the low DOC concentration ranges used in our experiments.
Our results confirmed that the fate of organic pollutants in subsoil is governed (i) by their intrinsic properties and (ii) by the subsoil properties. Environmentally relevant concentrations of DOC had low impacts on the mobility of organic pollutants.