INTERACTIONS OF MICROORGANISMS AND SOIL DURING FENAMIPHOS DEGRADATION

Enhanced degradation of a number of pesticides occurs in soils with a history of previous exposure to the chemical. Little is known, however, about the duration of enhanced degradation after a single field application of a pesticide or the microorganisms responsible for the enhancement. Soil samples collected 2, 3, and 4 yr after one application of the nematicide fenamiphos [ethyl-3-methyl-4-(methylthio)phenyl (1-methylethyl) phosphoramidate] at a rate of 4.48 kg ha-1 to an experimental site planted in potato (Solanum tuberosum L.) near Hastings, FL, were used to determine mineralization rates and total-toxic-residue (TTR) disappearance rates. Both mineralization rates and TTR disappearance rates. Both mineralization rates and TTR disappearance rates in samples collected 2 and 3 yr after field application were enhanced compared with the corresponding control samples. Half-lives for TTR in soil samples collected 2 and 3 yr after field application of fenamiphos were 22 and 89 d, respectively, while half-life values for the corresponding control samples were 131 and 130 d. During a 70-d incubation period, 67.2, 27.8, and 9.8% of applied C-14-fenamiphos was mineralized in soil samples collected 2, 3, and 4 yr after field application while 10.8, 11.5, and 9.5% of applied C-14-fenamiphos in the corresponding control samples was mineralized. Mineralization rates in soil collected 4 yr after field application were the same as in the control sample. Microorganisms capable of degrading fenamiphos could not be isolated from the soils. A mixed bacterial culture derived from soil collected 2 yr after the field application, in the presence of a small amount of this soil, however, did mineralize fenamiphos. Without the small amount of soil, the mixed culture did not mineralize the nematicide.