USE OF ISOPROTURON, ALONE AND IN COMBINATION WITH OTHER COMPOUNDS, POSTEMERGENCE IN WINTER AND SPRING, ON WINTER-WHEAT AND WINTER BARLEY .2. SIDE-EFFECTS ON DEHYDROGENASE-ACTIVITY, NITROGEN TRANSFORMATION AND STRAW DECOMPOSITION IN THE SOIL

The effect of isoproturon, alone and combined with dinoseb acetate and bifenox, on dehydrogenase activity, nitrogen transformations and straw decomposition following post-emergency application in winter and spring to winter wheat and winter barley was studied over two years at two sites. Isoproturon caused limited transient stimulation and inhibition of dehydrogenase activity, maximum 40%, and nitrogen turnover, 70-90%. Dinoseb acetate reduced general metabolic activity by up to 30%. Ammonium and nitrite concentrations were at times more than 100% higher than control values. In the laboratory, with temperature, soil moisture and sampling times similar to those in the field, isoproturon caused reductions in dehydrogenase activity in only a few cases but dinoseb acetate reduced the activity by up to 50%. The herbicide effects were clearer in a sandy than in a clay soil. The effect of time of application on the response of soil microbial activity to herbicides was evident only in the field.