Interactions of Microorganisms and Soil during Fenamiphos Degradation

Abstract

AbstractEnhanced 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 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 14C‐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 14C‐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.