Leaching of the organophosphorus nematicide fosthiazate.

Fosthiazate is an organophosphorus nematicide which was recently included in Annex I of the Directive 91/414/EEC under the clause that it should be used with special care in soils vulnerable to leaching. Thus, the leaching of fosthiazate was investigated in columns packed with three different soils which represented situations of high (site 2), intermediate (site 1) and low (site 3) leaching potential. The recommended dose of fosthiazate was applied at the surface of the soil columns and fosthiazate fate and transport was investigated for the next two months. Fosthiazate concentrations in the leachate collected from the bottom of the columns packed with soil from site 2 exceeded 0.1 microgl(-1) in most cases. This soil was characterized as acidic, indicating longer fosthiazate persistence, with low organic matter content, indicating weak adsorption, thus representing a situation vulnerable to leaching. In contrast, the lowest concentrations of fosthiazate in the leachate were evident in the columns packed with soil from site 3. This soil was characterized as alkaline, indicating faster degradation, with higher organic matter content, indicating stronger adsorption, thus representing a situation not favoring leaching of fosthiazate. The highest concentration of fosthiazate in the leachate from the columns packed with soil from site 2 was 3.44 microgl(-1) compared to 1.17 and 0.16 microgl(-1), which were the corresponding maximum values measured in columns packed with soil from sites 1 and 3, respectively. The results of the current study further suggest that fosthiazate is mobile in soil and can leach under conducive soil conditions like acidic soils with low organic matter content.

Influence of soil physicochemical and biological properties on the degradation and adsorption of the nematicide fosthiazate.

The degradation and adsorption of the organophosphorus nematicide fosthiazate were investigated in nine soils with various physicochemical and biological characteristics. Fosthiazate was more persistent in acidic soils (pH <6), with half-life (t1/2) values ranging from 53.3 to 57.7 days, compared to soils with higher pH (pH >7), with t1/2 ranging from 14.1 to 20.7 days. Application of antibacterial and antifungal antibiotics to soil samples resulted in a significant inhibition of fosthiazate degradation only in two of the three acidic soils. In contrast, soil autoclaving resulted in doubling the t1/2 of fosthiazate in all studied soils, suggesting that both microbial and abiotic processes contribute to fosthiazate degradation. Statistical analysis indicated a significant negative correlation (P < 0.01) between soil pH and t1/2. Fosthiazate was generally weakly adsorbed with Freundlich adsorption coefficient (Kf) values ranging from 1.23 to 2.74 mL/g. Fosthiazate concentration was strongly correlated with soil organic matter content with higher Kf values in soils with higher organic matter content (P < 0.01). The mean t1/2 and Kf values derived from the laboratory studies were used to parametrize the FOCUS groundwater (GW) models PRZM, PELMO, PEARL, and MACRO for nematicide application in potato and tomato crops. Predicted environmental concentrations produced by the models PEARL and MACRO suggested a potential risk for GW in several scenarios, unlike PELMO and PRZM, which predicted low risk for GW. These findings suggest that the environmental fate of fosthiazate is strongly influenced by soil characteristics and that this nematicide should be used with care in acidic, light soils with low organic matter content.