Study of atrazine fate in silty loamy soils of the Paris Basin via a combination of isotopic and pyrolytic methods

Introduction The fate of organic micro-pollutants in soil largely depends on their interactions with soil organic matter (SOM). Indeed, the intensity and nature of such interactions are major factors in the control of micro-pollutant bioavailability, and hence degradation and micro-pollutant mobility in the environment [Khan and Hamilton, 1980 ; Calderbank, 1989 ; Dec et al., 1990, 1997 ; Barriuso and Koshinen, 1996]. Residual micro-pollutants, i.e. the remaining micro-pollutants and their various transformation products (metabolites), occur in soil both in the water phase and associated with the solid phase. A part of these associated molecules, termed bound residues (BRs), cannot be released via extraction with water or organic solvents like methanol. SOM plays a major role in BR formation [e.g. Loiseau et al., 2000] and different types of interaction (like covalent bonds and trapping in organo-mineral aggregates) can be implied [Hsu and Bartha, 1976 ; Schiavon et al., 1977 ; Bollag et al., 1992 ; Dec and Bollag, 1997 ; Steinberg et al., 1987; Providenti et al., 1993]. The present study is concerned with the fate of atrazine in three silty loamy soils from the Paris Basin. Atrazine is largely used (ca. 5000 T/year in France) as herbicide for maize cropping and may cause important pollution problems through transfer to water resources. Experimental Incubations were performed for 60 days, using 14C labelled atrazine, with the selected soils. These incubations were carried out with 50 g of soil in closed vessels at room temperature in darkness. The production of 14C CO2 was regularly measured so as to determine the rate of atrazine mineralisation in the three incubated soils. These three soils exhibit large differences in pH and atrazine use (tabl. I). Following incubations, radioactivity measurements were performed on the different fractions isolated : aqueous extract, methanol extract, BRs in various size fractions (< 20 μm, 20–50 μm, > 50 μm). In addition, BR distribution was determined in the fulvic and humic acids and humin isolated from the < 20 μm fraction. In a second set of incubation experiments, the distribution of residual atrazine was examined in size fractions of organo-mineral aggregates separated by sieving in water (< 50 μm, 50–200 μm, 200 μm-1 mm, 1–2 mm, > 2 mm) and in free plant debris separated by floating. The GB1 soil was also incubated for 60 days using 13C labelled atrazine. Curie point flash pyrolysis combined both with isotopic me