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 pat hogens, is a common practice. However, there is a dearth of information on the side-effects of fungicides on key soil ecological processes. We investi gated 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 exper iments, one with captan and chlorothalonil and another with captan. In each laboratory batch incubation, soil was treated with a fungicide at approxim ately recommended field application rates (benomyl, 51 mg a.i. kg(-1), capt an, 125 m-g a.i. kg(-1) and chlorothalonil, 37 mg a.i. kg(-1)) and incubate d at 30 degreesC for 8 weeks. Some soils were amended with either ground al falfa leaves or ground wheat straw to provide additional substrates for soi l microorganisms and to alter rates of nitrogen mineralization/immobilizati on. All three fungicides suppressed the peak soil respiration in unamended soil by 30-50%, but the three fungicides had different effects in the amend ed soils. Soil dehydrogenase activity was stimulated by benomyl (18-21%) an d chlorothalonil (8-15%) except in the alfalfa amended soil, but was decrea sed by captan (40-58%) in both the straw-amended and unamended soils. The f ungicide-treated soils in general had less microbial biomass N concentratio ns than the untreated soils. Captan-treated soils had much higher NH4N conc entrations than the control soils, with or without the organic amendments. Benomyl and chlorothalonil had little influence on soil NH4N concentrations . Net N mineralization and nitrification rates were influenced by all fungi cide treatments as well as by the addition of organic materials. N minerali zation rates were significantly higher in captan-treated soils than in untr eated soils. N dynamics were influenced by chlorothalonil in a similar patt ern 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 mate rials added to the soil. The patterns of effects of the fungicides on soil nutrient cycling processes were not large and were specific to each fungici de. Captan appeared to have more pronounced overall effects on soil microbi al activity and nitrogen dynamics than either benomyl or chlorothalonil. (C ) 2001 Elsevier Science Ltd. All rights reserved.