The fate of metamitron, a sugar beet herbicide, in soil was studied in
laboratory and in field lysimeters under different water regimes. Lab
oratory degradation followed first-order kinetics with half-lives rang
ing from 18.8 to 56.3 d with different incubation conditions. Lysimete
r dissipation was consistently rapid with half-lives ranging from 4.4
to 8.0 d under the different water regimes. The influence of temperatu
re on laboratory degradation was within the range reported for many ot
her pesticides, while that of soil moisture was lower than for other p
esticides. Adsorption was well described by an L-type isotherm with a
1/n value far from unity in common with most polar herbicides. Metamit
ron movement through the lysimeters varied with the different amounts
of water supplied. Metamitron reached 60 cm in depth at day 7 after tr
eatment and 120 cm in depth at day 14 after treatment in lysimeters th
at received normal water amounts and about 10% of pesticide moved belo
w 120 cm in lysimeters that received, in addition to the normal water,
151 mm water at day 4 after treatment, which simulates a probable spr
ing 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 da
ys) of the pesticide in the lysimeters by means other than leaching, p
artially reduces this risk. ((C) Inra/Elsevier).