Nitrogen Transformations in Soil as Affected by the Fungicides Benomyl, Dyrene, and Maneb

The frequency and extent of fungicide use on putting green turfgrasses prompted investigations to determine the effect of three commonly used fungicides on N transformations in soil. Laboratory and field studies were conducted to study the effect of the following fungicides on nitrification and N mineralization in soil: benomyl [(methyl l-(butylcarbamoyl)-2-benzimidazolecarbamate)], rene [(2,4-dichloro-6-(0-chloranilino)-s-triazine)], neb (manganese ethylenebisdithiocarbamate). In laboratory studies, the three fungicides were added at rates of 0, 25, 75, and 150 ppm and the soil incubated at 21±1 C and moisture content of 30±1%. Analyses for NH⁺₄-N and (NO⁻₂ + NO⁻₃)-N were conducted at various times during a 16-week incubation period. Almost no effect was detected for benomyl, but a complete blockage occurred at the 150 ppm rate of maneb. Dyrene had an inter, mediate effect. Based on concentrations of (NH⁺,₄ + NO⁻₂ + NO⁻₃)-N in the untreated control as compared to treated soil, benomyl was stimulatory to N mineralization. Inhibitory effects were observed for both Dyrene and maneb after 4 weeks of incubation, but these effects had disappeared after 16 weeks. In the field, 14 weekly applications of benomyl, Dyrene, and maneb at 90, 90, and 135 g active ingredient/are were made to a ‘Penncross’ creeping bentgrass, Agrostis palnstris Huds., golf green. Soil samples were taken from a depth of 0 to 2.5 cm (excluding thatch) and determinations NH+4-N and (NO⁻₂ + NO₃⁻)-N concentrations were made at weekly intervals during a 5-week incubation. There was no effect of any of the fungicides on nitrification but enhanced N mineralization occurred with application of all three of the fungicides. The differences in the effects of these fungicides on nitrification and N mineralization in laboratory as compared to field applications were considered to be the result of lower rates of application associated with more rapid rates of degradation under field conditions as contrasted with the high single rates of application under laboratory conditions.