Influence of pesticides on nitrogen transformations in soil

Laboratory experiments were conducted to determine the effects of 2 concentrations of 53 pesticides on nitrogen transformations in a sand (0.8% C, pH = 5.3) and a silt (1.6% C, pH = 5.7). Tests were run for a maximum of 90 days, and the rates and quantities of ammonium, nitrate and nitrite were measured. For purposes of evaluation, differences between treated and control samples of more than 10% were considered a pesticide induced effect. When applied at the recommended doses, 31 pesticides, including 15 insecticides, 10 fungicides, and 6 herbicides had no long term (greater than 90 day) influence on soil nitrogen mineralization. At the end of tests with some pesticides, more nitrate had accumulated in treated than in control soils. This was attributed to brief and transitory stimulations of nitrate mineralization during the degradation of dead microbial cells. Since excess nitrate in treated soils was not related to an irreversible stimulation of nitrogen mineralization rates, it was evaluated as an artifact of the laboratory system used for the tests. In nitrification tests, the recommended dose of 53 pesticides, including 24 insecticides, 14 fungicides and 15 herbicides had no long term effects on nitrification rates. With some pesticides, less nitrate was formed in treated soils than in the controls. This was found to be due to brief and reversible reductions in the rates of nitrification. Since nitrite did not accumulate in any of the soils, none of the pesticides had a meaningful effect on nitrification. In nitrogen fixation tests, which ran a maximum of 10 days, 16 pesticides, including 7 insecticides, 4 fungicides and 5 herbicides, had no real influence on this process. Applied in the field as recommended, none of the pesticides tested in this work would have a relevant effect on nitrogen transformations.