A microcosm approach to assess the effects of fungicides on soil ecological processes and plant growth: comparisons of two soil types

The effects of three broad-spectrum fungicides, benomyl, captan and chlorothalonil, applied at recommended field application rates and 10 times these rates, on soil microbial activity and biomass (SIR), nitrogen dynamics, organic matter decomposition, and plant growth, were studied in microcosms containing two different types of soils. Soil type significantly affected all measurements except NH4–N, DON and in situ NO3–N availability measured on an anion-exchange membrane. The higher rate of all fungicide treatments had greater effects on soil microbial activity and nitrogen dynamics than the lower rates. Both concentrations of benomyl increased peak SIR rates and soil dehydrogenase activity (DHA). Captan and chlorothalonil decreased both peak SIR rates and DHA activity. Both application rates of benomyl and chlorothalonil did not affect concentrations of NH4–N in either soil; however, the higher rate of captan increased NH4–N concentrations significantly in both soils. Concentrations of NO3–N were increased by the higher application rates of both benomyl and chlorothalonil, but decreased by the higher application rate of captan. Dissolved organic N concentrations were increased by the higher application rate of fungicides. All fungicide treatments except the higher rate of chlorothalonil, enhanced rates of net N mineralization and nitrification initially, but reduced the rates after 20 days. Captan, especially at the higher application rate, decreased in situ concentrations of NO3–N significantly. The germination success of oat seeds was not influenced by any fungicide treatment in either type of soil. The lower application rate of benomyl stimulated oat growth significantly (0.05<P<0.10) in the silt loam soil, but the higher application rate of benomyl had the opposite effect. Neither benomyl nor chlorothalonil influenced rates of organic matter decomposition. The effects of captan on rates of organic matter decomposition depended on the soil type. The three fungicides differed in their effects on soil processes and plant growth in the two soil types. Compared to the relatively transient effects of benomyl and chlorothalonil, captan had a greater and longer-lasting overall influence on soil microbial activity and nitrogen dynamics.