Development and duration of accelerated degradation of nematicides in different soils

The development of accelerated degradation of nematicides was studied in incubation experiments with soils collected from trial plots at three long-term experimental fields. The plots had been treated annually for periods of 3–10 years with aldicarb, oxamyl, ethoprophos, fenamiphos or 1,3-dichloropropene (1,3-D). At one site (sandy soil; pH 7.3) highly accelerated degradation rates were measured for aldicarb, oxamyl and ethoprophos in soil from plots treated three times. After 5 years without further treatments, these high degradation rates were still present for aldicarb and oxamyl, but not for ethoprophos. At another site (sandy soil; pH 5.6) little or no increase in the degradation rates of aldicarb, oxamyl and ethoprophos was measured in soil from plots treated 10 times. Three annual applications of fenamiphos to a loamy soil (pH 7.7), doubled the rate of loss of the total toxic residue compared with the untreated soild. Fenamiphos and oxamyl did not show cross-enhancement when incubated in ethoprophos-adapted soils. After a lag-time of 4–9 days degradation of 1,3-D was very rapid in all soils including controls which had been treated only once or twice well before the sampling dates. Seven annual applications of 1,3-D resulted in only slightly shorter lag-times. An interval of 5 years without treatment resulted in only slightly longer lag-times. Simultaneous incubation of a commercial dose of the fumigant methyl isothiocyanate inhibited the fast degradation of 1,3-D.