Effect of the insecticide carbofuran on microbial activities in flooded soil

The effects of carbofuran on some soil microbial activities—nitrogen mineralization, nitrification, nitrogen-fixation by blue-green algae, and urea hydrolysis—were investigated in the laboratory and the greenhouse. The addition of 10μg a. i. carbofuran per gram dried soil had no inhibitory effect on the mineralization of native soil nitrogen. Nitrifying activity was enhanced in flooded soil treated with 10, 20, 50, and 100ppm a. i. carbofuran. Nitrifying activity increased with increasing carbofuran concentration. The growth of blue-green algae was promoted by the addition of 6kg a. i. carbofuran/ha to the floodwater. Subsequently, a marked increased in the acetylene reduction activity of carbofuran-treated floodwater was obtained. Turbidity and an abundance of green algae distinguished the untreated floodwater from the carbofuran-treated one. The positive effect of carbofuran on phototrophic N2-fixation appeared after its decomposition. The addition of up to 15ppm a. i. (μg/ml floodwater) carbofuran had no effect on the acetylene-reduction activity of Gloeotrichia sp., but 20ppm a. i. caused a significant lowering of that activity. The rate of urea hydrolysis appeared to be faster in dryland than in flooded soil. The addition of 50ppm a. i. carbofuran did not affect the rate of urea hydrolysis.