Effects of the insecticide endosulfan on nitrification in low pH agricultural soils

Two methods of measuring nitrification in soil bioassays were compared for their sensitivity in estimating toxic effects of endosulfan, a cyclodiene insecticide. Both technical grade endosulfan (95% pure) and endosulfan formulated as a commercial preparation (Thiodan 4EC) were tested in a sandy loam (pH 5.7), a silt loam (pH 5.0), and a clay loam (pH 4.9) soil. Nitrification was assayed using both soil perfusion and batch incubation techniques. There was a significant difference in the nitrification pattern evidenced by the three soils. In the sandy loam soil up to 90% of the added ammonium ion was converted into nitrate within 30 days, while the silt loam required 40 days to achieve similar nitrate levels. Activity in the clay loam was very slow and only 5% of the added ammonium ion was converted into nitrate within 70 days. The nitrification pattern for any given soil was similar when measured by both test methods. At insecticide levels of 10, 50, and 100 ppm of active ingredient, there was no significant difference in toxicity between the commercial preparation and technical endosulfan in 50% of the systems tested. In the remainder, the commercial formulation was usually more toxic. For concentrations of 500 and 1000 ppm of active ingredient, endosulfan supplied as the commercial preparation was consistently more toxic than the technical material when tested toward nitrification. There was also a significant difference between the two incubation methods in their sensitivity to endosulfan. The exact level of toxicity was dependent upon both the soil type and the incubation method used. Generally, endosulfan was more toxic with the batch incubation system and in the silt loam soil. The importance of these results in nitrification bioassays are discussed.