Effects of the fungicides benomyl, captan and chlorothalonil on soil microbial activity and nitrogen dynamics in laboratory incubations

The use of fungicides in agriculture, to protect plants from soil-borne pathogens, is a common practice. However, there is a dearth of information on the side-effects of fungicides on key soil ecological processes. We investigated the effects of three fungicides, benomyl, captan and chlorothalonil, on soil microbial activity (substrate-induced respiration and dehydrogenase activity), and nitrogen dynamics (NH4–N and NO3–N) in two laboratory experiments, one with captan and chlorothalonil and another with captan. In each laboratory batch incubation, soil was treated with a fungicide at approximately recommended field application rates (benomyl, 51 mg a.i. kg−1, captan,125 mg a.i. kg−1 and chlorothalonil, 37 mg a.i. kg−1) and incubated at 30°C for 8 weeks. Some soils were amended with either ground alfalfa leaves or ground wheat straw to provide additional substrates for soil microorganisms and to alter rates of nitrogen mineralization/immobilization. All three fungicides suppressed the peak soil respiration in unamended soil by 30–50%, but the three fungicides had different effects in the amended soils. Soil dehydrogenase activity was stimulated by benomyl (18–21%) and chlorothalonil (8–15%) except in the alfalfa amended soil, but was decreased by captan (40–58%) in both the straw-amended and unamended soils. The fungicide-treated soils in general had less microbial biomass N concentrations than the untreated soils. Captan-treated soils had much higher NH4 N concentrations than the control soils, with or without the organic amendments. Benomyl and chlorothalonil had little influence on soil NH4 N concentrations. Net N mineralization and nitrification rates were influenced by all fungicide treatments as well as by the addition of organic materials. N mineralization rates were significantly higher in captan-treated soils than in untreated soils. N dynamics were influenced by chlorothalonil in a similar pattern to captan but reached peak nitrification rates earlier (day 7), in the alfalfa-amended soil. The effects of the three fungicides on soil microbial activity and nitrogen dynamics depended on the quality of the organic materials added to the soil. The patterns of effects of the fungicides on soil nutrient cycling processes were not large and were specific to each fungicide. Captan appeared to have more pronounced overall effects on soil microbial activity and nitrogen dynamics than either benomyl or chlorothalonil.