Dry-wet cycles increase pesticide residue release from soil.

Soil drying and rewetting may alter the release and availability of aged pesticide residues in soils. A laboratory experiment was conducted to evaluate the influence of soil drying and wetting on the release of pesticide residues. Soil containing environmentally long-term aged (9-17 years) (14) C-labeled residues of the herbicides ethidimuron (ETD) and methabenzthiazuron (MBT) and the fungicide anilazine (ANI) showed a significantly higher release of (14) C activity in water extracts of previously dried soil compared to constantly moistened soil throughout all samples (ETD: p < 0.1, MBT and ANI: p < 0.01). The extracted (14) C activity accounted for 44% (ETD), 15% (MBT), and 20% (ANI) of total residual (14) C activity in the samples after 20 successive dry-wet cycles, in contrast to 15% (ETD), 5% (MBT), and 6% (ANI) in extracts of constantly moistened soils. In the dry-wet soils, the dissolved organic carbon (DOC) content correlated with the measured (14) C activity in the aqueous liquids and indicated a potential association of DOC with the pesticide molecules. Liquid chromatography MS/MS analyses of the water extracts of dry-wet soils revealed ETD and MBT in detectable amounts, accounting for 1.83 and 0.01%, respectively, of total applied water-extractable parent compound per soil layer. These findings demonstrate a potential remobilization of environmentally aged pesticide residue fractions from soils due to abiotic stresses such as wet-dry cycles.