Sorption and Degrading Parameters for Modeling Nematicide Fate in Soil

Lee, Chee Chow
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Since the ban of the traditional fumigants, DBCP and EDB, for nematode control in pineapple, there has been considerable interest in a non-volatile organosphosphorus nematicide, fenamiphos [Nemacur®, ethyl 3-methyl-4-(methylthio)phenyl (1-methylethyl) phosphoramidate]. The fate of fenamiphos is unknown in diverse field soils in Hawaii. The ability of mathematical simulation models to predict movement and persistence of fenamiphos in soils from key input parameters associated with mathematical descriptions of degradation, sorption and leaching, will aid us in designing optimum management strategies to achieve maximum efficacy and minimize environmental contamination. In view of the proposed modeling efforts, experiments were conducted to evaluate the errors and uncertainties in measurements of two important processes, sorption and degradation. Fenamiphos sorption measurements were conducted by the conventional batch method on Molokai and Pane soils under aerobic conditions. The impact on measured sorption by (1) degradation of fenamiphos during equilibration, (2) variable moisture status of soils before measurements and (3) equilibration time were evaluated. Apparent sorption and sorption corrected for degradation at both 4 and 24 hours equilibration on both soils increased in the following order: field-moist < prewetted < air-dried. Degradation of fenamiphos to fen. sulfoxide during batch equilibration occured at all moisture treatments, and the impact of degradation (45% of fenamiphos degraded) on sorption measurement was significant on the Pane soil. On the Molokai soil however, the impact was negligible because less than 20% of fenamiphos degraded. Competition of water molecules with fenamiphos for sorption sites and fenamiphos degradation during equilibration probably accounted for the differences in sorption due to pre-sorption moisture status. The percentage differences in fenamiphos sorption betweet. air-dried and field-moist soils after 24 hours equilibration were 34 % and 37% for Pane and Molokai soils, respectively. The effect of initial moisture on sorption measurements may not be of practical importance when we consider that the average coefficient of variation of fenamiphos and fen. sulfoxide sorption determined on soils obtained from nine fields was 35% and 46%, respectively. In the degradation study, the reliability of using laboratory-generated degradation rates for fenamiphos and fen. sulfoxide to predict degradation of these nematicides under field conditions was evaluated. Field and laboratory methods of determining fenamiphos degradation were compared at six locations within two cropped pineapple fields on the Oahu Dole and Del Monte plantations: the methods were compared using first-order degradation rates and the quantity of fenamiphos residue remaining at a given time. A similar comparison with fen. sulfoxide was performed on the Del Monte field. In field experiments, nematicides were applied to insitu field cores contained in aluminum cylinders (inserted into the tilled layer of pineapple beds) at six locations per field. Laboratory experiments were performed by application of 14C-labeled nematicides to soils collected from the same locations as the field tests; treated soils were incubated under controlled temperature and moisture. On the Del Monte field, the field and laboratory degradation data were fit reasonably well by first-order kinetics. First-order degradation rates determined from laboratory and field methods were similar. Fenamiphos degradation data from the Dole field generally deviated statistically from first-order kinetics; the better statistical resolution of deviations from first-order kinetics on the Dole field was largely due to q much lower overall standard error for this field. The difference, however, between laboratory and field measured fenamiphos residues on the Dole field was within a factor of 1.4. In view of the uncertainties in degradation measurements contributed from field soil variability and analytical techniques, this differrence is tolerable. Degradation rates of fenamiphos and fen. sulfoxide determined from laboratory methods are therefore considered reliable estimates for modeling persistence of these chemicals in field soils under pineapple cultivation.
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