Degradation of the nematicide oxamyl under field and laboratory conditions

Abstract

The persistence of nematicides such as oxamyl can vary greatly in field conditions. The objectives of the present studies were: i) to compare oxamyl degradation in soils with different properties; ii) to quantify and examine the influence of various abiotic factors on oxamyl degradation; iii) to establish the validity of using simulated models to predict the degradation in the field; and iv) to examine if a second application of oxamyl to the same soil 13 or 26 weeks after the first application enhances degradation. The first two studies included field measurements of oxamyl concentration and parallel laboratory incubations. For the field measurements, soils were collected from each of ten potato (Solanum tuberosum) fields in Shropshire, UK, immediately after application of oxamyl on the day of planting and then at weekly intervals for the duration of the two experiments. After each collection, oxamyl was extracted and its concentration determined. For the laboratory incubations, soils were collected from the same ten sites immediately prior to field application and received one application of oxamyl in the laboratory at the same day (day 0). The PERSIST model was then used to predict oxamyl degradation in the field (modelled degradation). Modelled degradation was then compared with the measured degradation up to 91 days (study 1) or 56 days (study 2) after application. In study 3, an extra application of oxamyl to that in the field at day 0 was made in the laboratory at 13 or 26 weeks after application. There were wide variations in the persistence of oxamyl between the ten sites, with the field half-life ranging from 10 to 24 days. Degradation in the field was significantly greater at site 4, where it could not be detected 28 days after application. At other sites, the chemical persisted for 42-63 days and was still detectable at two sites 91 days following application. Soil temperatures had a greater impact on oxamyl degradation than rainfall accounting for up to a maximum of 79% of the variation. The short persistence at site 4 was attributed to the combination of warm and moist conditions in a higher pH soil. The PERSIST model predicted the same rate of decline of oxamyl as actually occurred in the field at only four (sites 5, 6, 7 and 8) sites. At the other sites, degradation in the field occurred at more rapid rates than predicted. This could be as a result of the model not allowing for the movement of nematicide by leaching, or because enhanced degradation of nematicides occurred at these latter sites, or due to a combination of these factors. The wide variation in half-lives and the behaviour of soils after subsequent additions of oxamyl in study 3 were suggestive of complex microbial dynamics even under controlled conditions. Further studies would be required to establish the influence of soil microflora together with that of abiotic parameters on oxamyl degradation.