A microcosm approach for evaluating the effects of the fungicides benomyl and captan on soil ecological processes and plant growth

The effects of benomyl and captan on soil ecological processes were tested in integrated terrestrial microcosms containing agricultural soil, organic amendments and wheat seedlings. The effects of the two fungicides on important soil ecological processes were evaluated by measuring soil microbial activity and biomass, including soil substrate-induced respiration (SIR), soil enzyme activity (dehydrogenase, urease and acid phosphatase) and microbial biomass nitrogen concentrations; nitrogen dynamics, including extractable inorganic nitrogen, dissolved organic nitrogen concentrations, net N mineralization and nitrification rates; rates of organic matter decomposition, using chopped wheat straw; in situ inorganic nitrogen concentrations using ion-exchange resin bags, and plant growth. The quality of the organic amendments (ground alfalfa leaves or chopped wheat straw) influenced the effects of the two fungicides on soil microbial processes and nitrogen availability strongly. Rates of SIR, soil enzyme activities (except urease activity), microbial biomass N and dissolved organic N concentrations were all decreased significantly by the fungicide treatments. Rates of wheat straw decomposition were also inhibited by the fungicide applications. Soil urease activity, NH4+-N and NO3--N concentrations, and initial net N mineralization and nitrification rates were increased by the fungicide treatments. In situ concentrations of NH4+-N and NO3--N in ion-exchange resin bags differed between the two fungicide treatments. Captan increased amounts of NH4+N and NO3--N in the resin bags significantly, compared to those in the untreated controls, or in benomyl-treated soils. The germination success of wheat seeds after 7 days, plant biomass (shoot + roots) as well as the total nitrogen uptake, were all increased by the captan treatment. The two fungicides differed in their effects on some soil processes and plant growth, as well as on rates of nitrogen uptake by plants; captan having a greater and long-lasting overall influence than benomyl. We concluded that these integrated microcosm techniques and resultant data can provide a better understanding of the interactions between fungicide applications and soil ecological processes than single investigations of the individual processes. (C) 2001 Elsevier Science B.V. All rights reserved.