Enzymatic and biological assessment of sulfonylurea herbicide impact on soil bacterial communities

Soil microorganisms play an important role in the decomposition of organic and inorganic matter in soil, being capable of breaking down different types of xenobiotic substances in soil. Application of herbicides in crops is commonly used in agriculture to increase agricultural productivity and weed destruction. Two sulfonylurea herbicides (tribenuron methyl and nicosulfuron) were applied to the soil samples in different doses. The effect of the herbicides on soil enzymatic activities and on the main groups of microorganisms involved in the soil nitrogen cycle (ammonifying, nitrifying and denitrifying bacteria) was studied under laboratory conditions. As far as the values of enzymatic activities are concerned, it was determined that they were strongly inhibited by the application of these two herbicides. Values of the dehydrogenase and urease activities from the experimental variants, most closely reflects the inhibitory effect of herbicides. Enzymatic indicator of soil quality shows values recorded between 0.390 to 0.541 for soils treated with tribenuron-methyl, respectively 0.401 to 0.431 in those treated with nicosulfuron. The results showed that nicosulfuron had a lower inhibitory effect on the enzymatic activity of soil from tribenuron-methyl. Ammonifying, nitrifying, and denitrifying bacteria responded differently to the action of herbicides, depending on the dose that was applied. Bacterial indicator of soil quality recorded values between 1.645 to 1.706 for soils treated with tribenuron-methyl and 1.651 to 1753 in those treated with nicosulfuron. Tribenuron-methyl and nicosulfuron affect negative bacterial metabolic activity and bacterial communities.   Key words: Nicosulfuron, tribenuron-methyl, soil, enzymatic activity, bacteria nitrogen cycle.