Response of microorganisms and enzymes to soil contamination with a mixture of terbuthylazine, mesotrione, and S-metolachlor

The research objective has been to evaluate the effect, unexplored yet, of a mixture of three active ingredients of the herbicide Lumax 537.5 SE: terbuthylazine (T), mesotrione (M), and S-metolachlor (S) on counts of soil microorganisms, structure of microbial communities, activity of soil enzymes as well as the growth and development of maize. The research was based on a pot experiment established on sandy soil with pHKCl 7.0. The herbicide was applied to soil once, in the form of liquid emulsion dosed as follows: 0.67, 13.4, 26.9, 53.8, 108, 215, and 430 mg kg-1 of soil, converted per active substance (M + T + S). The control sample consisted of soil untreated with herbicide. The results showed that the mixture of the above active substances caused changes in values of the colony development (CD) indices of organotrophic bacteria, actinomycetes, and fungi and ecophysiological diversity (EP) indices of fungi. Changes in the ecophysiological diversity index of organotrophic bacteria and actinomycetes were small. The M + T + S mixture was a strong inhibitor of dehydrogenases, to a less degree catalase, urease, β-glucosidase, and arylsulfatase, while being a weak inhibitor of phosphatases. The actual impact was correlated with the dosage. The M + T + S mixture inhibited the growth and development of maize. The herbicide Lumax 537.5 SE should be applied strictly in line with the regime that defines its optimum dosage. Should its application adhere to the manufacturer's instructions, the herbicide would not cause any serious disturbance in soil homeostasis. However, its excessive quantities (from 13.442 to 430.144 mg kg-1 DM of soil) proved to be harmful to the soil environment.