Soil behaviour of metamitron in laboratory and lysimeter studies

The fate of metamitron, a sugar beet herbicide, in soil was studied in laboratory and in field lysimeters under different water regimes. Laboratory degradation followed first-order kinetics with half-lives ranging from 18.8 to 56.3 d with different incubation conditions. Lysimeter dissipation was consistently rapid with half-lives ranging from 4.4 to 8.0 d under the different water regimes. The influence of temperature on laboratory degradation was within the range reported for many other pesticides, while that of soil moisture was lower than for other pesticides. Adsorption was well described by an L-type isotherm with a 1/n value far from unity in common with most polar herbicides. Metamitron movement through the lysimeters varied with the different amounts of water supplied. Metamitron reached 60 cm in depth at day 7 after treatment and 120 cm in depth at day 14 after treatment in lysimeters that received normal water amounts and about 10% of pesticide moved below 120 cm in lysimeters that received, in addition to the normal water, 151 mm water at day 4 after treatment, which simulates a probable spring rain event in northern Italy. The marked movement of the pesticide in these conditions suggests that it is to be considered a potential pollutant in irrigated crops but, the rapid disappearance (DT50 4-8 days) of the pesticide in the lysimeters by means other than leaching, partially reduces this risk. ((C) Inra/Elsevier).