Soil fumigation alters adsorption and degradation behavior of pesticides in soil.

Many crops are produced using soil fumigation and chemical pesticides to control soil-borne fungi and bacterial diseases, nematodes and weeds. Fumigation of soil, however, may alter its ability to adsorb, degrade and volatilize pesticides, which can then change the potential for pesticides to leach into groundwater. Soil adsorption kinetics, Freundlich isothermal adsorption and pesticide degradation techniques were used to determine the potential for pesticides to pollute groundwater in fumigated soil. The effect on soil pesticide adsorption in different types of chloropicrin (CP) fumigated soils was also examined. We observed that the equilibrium adsorption (qe) decreased significantly at 24 h. Soil fumigation decreased the Freundlich Kf and Kfoc values, and increased the Freundlich exponent 1/n values, for pesticides in fumigated Beijing soil. Soil fumigation influenced the Kf of pendimethalin, oxyfluorfen and abamectin the most, which themselves had a larger Kf in untreated soil. This indicated that the greater the soil pesticide adsorption the greater the influence of a fumigation treatment on that pesticide. The Kf was decreased more in the Heilongjiang and Beijing CP-fumigated soils that had high organic carbon content compared to Hunan soil. Fumigation of the soil with CP extended the half-life values of fosthiazate (from 34.3 to 43.1 days) and azoxystrobin (from 52.9 to 64.2 days), which increased their potential to leach into groundwater. Famers should minimize the quantity of some pesticides applied to fumigated soil, or apply some pesticides 60 days after fumigation, in order to avoid ground water pollution when crops are grown in fields or greenhouses.