| Summary: | The fate of the herbicides isoproturon (formulated as Arelon) and 3,6-dichloropicolinic acid (formulated as Lontrel) and their effects on several microbiological populations and processes in soil and in the wheat rhizosphere were examined under field, glasshouse and laboratory conditions. Effects on pure microbial cultures were also studied. The herbicides took 1 to 6 weeks to reach 50% of their initial concentrations in soil. 14C-labelled isoproturon was transformed in soil, with or without wheat roots, by demethylation, hydrolysis and hydroxylation to release the aniline derivative. There were no significant 14C-containing soil bound residues. Arelon applied to field soil in autumn at 2.5 kg a.i. ha-1 (normal rate) and Lontrel sprayed late in the following spring on winter wheat (Zadok's code 19, 29, 30) at 0.625 kg a.e. ha -1 (5 times recommended rate) caused variable effects including occasional, transient increases and decreases in numbers of bacterial and fungal propagules but no changes in levels of NH4+-N, NO2--N, NO3--N or PO43- in soil. Inconsistent effects, without distinct trends, were also recorded on populations in planted soil during a glasshouse experiment using soil-applied Arelon (0.75 kg a.i. ha-1) and in laboratory - incubated soil treated with either Arelon (1.3 and 6.3 mug a.i. g-1 oven-dry soil) or Lontrel. (0.06 and 0.31 mug a.e. g-1 oven-dry soil). Although in the former study, populations of fluorescent pseudomonads and total bacteria in the rhizosphere were affected for long periods. Growth of pure cultures of bacteria and fungi was not prevented by Lontrel (0.1 to 10.0 mug a.e. ml -1). Arelon (1.0 to 60.0 mug a.i. ml-1) did not affect bacteria but at the highest concentration (about 50 times field rate) there was a decrease in colony growth and pigmentation of some fungi. The results suggest that Arelon and Lontrel, in practical use, are unlikely to have harmful effects on soil micro-organisms and so on soil fertility.
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