The Effect of Sulphonylurea Herbicides on the Microbial Activity in Soil under Maize

Depeneding on the quality and quantity of herbicides and their mode of action and soil type, the number of different groups of microorganisms in soil increased or decreased. Sulphonylurea herbicides belong to a group of pesticides which in doses of only a few grams per hectar are sufficient to control weeds. The mode of action of these herbicides is based on blocking the enzyme acetolactate synthase (ALS, EC 4.1.3.18) which is involved in the biosynthesis of amino acids valin, leucine and isoleicine, thus making them non-toxic for animals but very toxic for sensitive plants and microorganisms. The aim of this investigation was to investigate the effect of Nicosulfuron and Rimsulfuron, sulphonylurea based herbicides, on the total number of bacteria, number of Azotobacter sp. and dehydrogenase activity in surrounding soil and rhizosphere under the maize crop. The total number of bacteria and the number of Azotobacter showed a stronger reaction to herbicide application than did dehydrogenase activity. The negative effect was the most conspicuous seven days after herbicide application because microbiological degradation of herbicides was at its lowest at that time. The changes in microbial activity were greater in the rhizosphere than in the surrounding soil, as the number of micro-organisms and dehydrogenase activity were higher in it.