Aerobic degradation assessment for the fungicide BAS 505 using batch and intact soil core methodologies

Although registration protocol stipulates that pesticide degradation be quantified using homogeneous soil, research suggests that degradation in intact soil may give results more consistent with field data. This project examined degradation of the turf and cereal fungicide BAS 505 [N-methyl-(E)-2-methoxyamino-2-(2-((2,5-dimethylphenoxy)methyl)phenyl) acetamide]. Yearlong and four-month-long incubation studies compared degradation rates in intact cores and homogeneous (batch) samples of Ruston fine sandy loam (fine-loamy, siliceous, thermic Typic Paleudults) soil. Recovery of BAS 505 in methanol extracts was measured 12 times over the 360 d incubation by HPLC-LSC analysis, and declined to 36 % and 57 % of the amount applied in cores and batch soils, respectively, by 360 d. But degradation in cores was faster than in batch soil only after long-term incubation. Since geostatistical surveys of soil biological, chemical and physical properties at the study site revealed spatial variability, a four month incubation using soil from different landscape positions was performed to verify field-wide consistency. Recovery was measured 5 times and after 120 d decreased to 65 % and 67 % of applied in cores and batch soils (averaged across all positions), respectively. This study found no significant difference in degradation of BAS 505, either between systems for any landscape position or among positions for intact and batch soils. In both studies and systems, degradation rate decreased over time and could be described by Nth-order kinetics but not 1st-order. Sorption BAS 505 in the Ruston soil was reversible so that sorption kinetics was likely not the cause of slowing degradation rate. Decreasing microbial activity with time (as by lack of nutrient inputs) may have occurred but this was not shown by the highly variable microbial biomass C data. A follow-up greenhouse study that compared BAS 505 degradation in packed cores of Ruston soil under bermudagrass (Cynodon dactylon (L.) Pers.), bare and bare but shaded found evidence that the presence of living grass led to faster degradation of BAS 505 versus bare soil. Thus, long-term static laboratory incubations may be a poor basis for projecting environmental fate and persistence