Plants affect the dissipation and leaching of anilide pesticides in soil mesocosms: Insights from compound-specific isotope analysis (CSIA)

Abstract

The soil-plant system can act as a prevailing zone of pesticide dissipation in agroecosystems, which may influence pesticide leaching following rainfall events. Here we examined the contribution of leaching, dissipation and degradation of widely used anilide pesticides in planted and unplanted mesocosms with contrasting vineyard and forest soils. The mesocosms were spiked at 25 mg/kg with acetochlor, alachlor, S-metolachlor, butachlor and metalaxyl, and followed-up for 75 days. Successive rainfalls were applied on days 45, 60 and 75 to collect pesticide leachates. Bulk soil and rhizosphere samples were collected to evaluate pesticide dissipation. Up to 14% of initially spiked pesticides was exported by leaching. The first rainfall accounted for 69–99% of the total pesticide leaching. Pesticide dissipation in soil mesocosms on day 75 ranged from 58 to 99% and was larger in planted than in unplanted mesocosms. Compound-specific stable isotope analysis (CSIA) of the pesticides showed that biodegradation in both the vineyard and forest soil mesocosms occurred mainly in the soil solution that was leached following rainfall events rather than in the bulk soil. Changes in carbon isotope values (Δδ13C up to 13.9 ± 0.5‰) of acetochlor, alachlor, S-metolachlor and butachlor in leachates were more pronounced in planted than in unplanted mesocosms, indicating predominant pesticide degradation in the planted mesocosms. Pesticide biodegradation was favoured in the soil solution of soil-plant systems, independently of the soil type. Leaching of the soil solution, pesticide ageing and biodegradation in the bulk soil and the rhizosphere were the main processes of pesticide dissipation. Overall, this study emphasizes the variability of leaching and degradation of anilide pesticides in agricultural soils, and proposes a framework using CSIA to examine the contribution of dissipation processes in soil-plant systems.