Effect of synthetic clay and biochar addition on dissipation and 1 enantioselectivity of tebuconazole and metalaxyl in an agricultural soil : 2 laboratory and field experiments

24 25 Laboratory and field experiments were conducted to assess how the addition of oleate26 modified hydrotalcite (clay) and biochar (BC) to an agricultural soil affected the sorption, 27 leaching, persistence, and enantiomeric composition of soil residues of two chiral fungicides, 28 tebuconazole and metalaxyl. Laboratory experiments showed that the sorption of both 29 fungicides ranked as follows: unamended soil < BC-amended soil < clay-amended soil. The 30 addition of clay at a rate of 1% increased metalaxyl soil sorption coefficient (Kd) from 0.34 31 to 3.14 L kg -1 and that of tebuconazole from 2.4 to 47.4 L kg -1 . In our experimental set-up, 32 field plots were either unamended or amended with clay (2 t ha -1 ) or BC (4 t ha -1 ), and 33 subsequently treated with a mixture of tebuconazole and metalaxyl at 3 and 6 kg ha -1 , 34 respectively. The leaching, persistence, and enantiomer composition of fungicides residues 35 was monitored by sampling at different soil depths (0-5, 5-10, 10-20 cm) for 98 days. No 36 significant changes in the scarce mobility and long persistence of tebuconazole upon 37 amending the soil with clay or BC were observed. In contrast, sorption to clay and BC 38 particles reduced the leaching and degradation of metalaxyl and the clay increased its 39 persistence in the topsoil compared to the unamended soil. The enantioselective analysis of 40 tebuconazole and metalaxyl soil residues indicated that tebuconazole remained mostly 41 racemic along the experiment, whereas for metalaxyl the concentration of S-enantiomer was 42 greater than the concentration of R-enantiomer, more so at longer experimental times and 43 deeper horizons. Nevertheless, for the top 0-5 cm soil layer metalaxyl remained more 44 racemic in clayand BC-amended soil than in unamended soil. Our results show that 45 addition of amendments with high sorptive capacities can be beneficial in reducing leaching 46 and degradation losses of chiral pesticide enantiomers from the topsoil, and that sorption by 47 the amendments can influence the final enantiomeric composition of pesticide residues. 48