Isomer-Selective Behavior of Metalaxyl in Organically-Amended Soil.

Addition of organic amendments can impact the enantioselective behavior of chiral pesticide enantiomers in agricultural soils. In this work, we investigated how amending a sandy loam agricultural soil with composted olive mill waste (OMWc) or its biochar (BC) at a rate of 2% (w:w) affected the enantioselectivity of the sorption, dissipation, and leaching processes of the fungicide metalaxyl. Sorption of metalaxyl was higher on BC than on OMWc and was not enantioselective in either case. Sorption of metalaxyl enantiomers followed the order BC-amended > OMWc-amended > unamended soil.  Both enantiomers showed greater resistance to desorption from BC-amended soil compared to unamended and OMWc-amended soil. Dissipation studies revealed that the degradation of metalaxyl was more enantioselective in unamended and OMWc-amended soil than in BC-amended soil. The leaching of both S- and R-metalaxyl was almost completely suppressed after amending the soil with BC. Less than 10% of the fungicide applied to columns of soil amended with BC was recovered in leachates, whereas higher percentages leached in unamended and OMWc-amended soil, the process being more enantioselective in the latter cases. Total recoveries of both enantiomers were greater for BC-amended soil columns than for unamended soil columns, indicating reduced degradation in BC-amended soil. Our findings show that addition of BC affected the final enantioselective behavior of metalaxyl in soil by reducing its bioavailability through sorption, and to a greater extent than OMWc. Enantioselectivity is altered even though sorption itself is non-enantioselective. BC could contribute to a reduction in losses of chiral pesticide enantiomers and mitigate the contamination problems associated with their use, and could also be useful as an immobilizing agent in soil remediation strategies.