Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/14/2006
Publication Date: 3/1/2006
Citation: Mcdonald, J.A., Gaston, L.A., Jackson, S.H., Locke, M.A., Zablotowicz, R.M. 2006. Degradation kinetics assessment for the fungicide BAS 505 in intact soil cores versus batch soils. Soil Science 171:239-248. Interpretive Summary: Evaluating pesticide degradation kinetics is an important component associated with pesticide registration, but sometimes conclusions drawn from assessement studies vary according to the methodology used. A laboratory study was conducted to compare methods of assessment (use of intact soil leaching cores versus batch mixture with soil) on the degradation kinetics of the fungicide BAS 505. It was determined that degradation did not differ between assessment methodologies until after 180 days, when fungicide degradation was faster in the soil cores, possible because of higher microbial populiations in soil cores. This information should be of interest to those involved in establishing protocols for pesticide use registration.
Technical Abstract: Apparent field degradation rates of pesticides are often faster than determined in the laboratory using homogeneous soil. This project developed an intact soil core method for determining aerobic degradation rate that is intended to address such discrepancies. The fungicide BAS 505 (14C-labeled), [N-methyl-(E)-2-methoxyamino-2-(2-((2,5-dimethylphenoxy) methyl)phenyl)acetamide], was applied to surface 0 to 7.5 cm Ruston (fine loamy, siliceous, thermic Typic Paleudults) soil cores (triplicate and duplicate series) and homogeneous (batch) soil in biometer flasks (triplicate). Recovery was measured 12 times over the 360 d incubation. Effect of BAS 505 on microbial biomass C was tracked using a triplicate series of untreated cores. Mineralization rate for cores, initially slower than for batch soil, increased to give more 14CO2 lost by day 360 (11 % and 8 % of applied, respectively). Unextractable 14C increased to 21 % for cores, similar to that for batch soil (18 %). Parent recovery in combined MeOH and MeOH:water extracts (HPLC-LSC analysis) decreased to 37 % and 36 % in triplicate and duplicate core series after 360 d whereas recovery from batch soil decreased to 57 %. Rate of degradation in both systems decreased over time and so could be described by Nth-order kinetics but not 1st-order. High sorption or lack of nutrient inputs may account for slow and decreasing degradation rates. However, effect of long-term incubation on deceasing microbial activity was not supported by the highly variable biomass C data. Despite lower recovery from cores, there was no significant difference between degradation rates in core and batch soils.