Effects offenpropimorph on bacteria and fungi during decomposition of barley roots L. Thirup a , K. Johnsen a, * , V. Torsvik b , N.H. Spliid c , C.S. Jacobsen a a Department of Geochemistry, Geological Survey of Denmark and Greenland, Thoravej 8, DK-2400 Copenhagen NV, Denmark b Institute of Microbiology, University of Bergen, Jahnebakken 5, N-5020 Bergen, Norway c Department of Crop Protection, Danish Institute of Agricultural Sciences, Research Centre Flakkebjerg, DK-4200 Slagelse, Denmark Received 29 March 2000; received in revised form 26 February 2001; accepted 12 March 2001 Abstract The study examined the effects of the fungicide fenpropimorph in the formulation Corbel) on primary decomposer organisms in soil. Bacterial and fungal succession was followed on dead young barley roots buried in fungicide-treated or untreated soil. Fenpropimorph was added to the soil in a concentration realistically achieved in ®eld topsoil when using the recommended dose. Over a 56-day period we measuredthelengthofactivefungalhyphae,theabundanceoftotalculturablebacteria,theabundanceoftwoculturablesubgroupsrelevant to the soil environment hyphae-forming actinomycetes and Pseudomonas), and the diversity of bacterial DNA. The soil fenpropimorph content followed ®rst order disappearance concomitant with the ®rst order formation of fenpropimorphic acid. Fenpropimorphinhibitedthegrowthofactivefungiduringthe®rst10days,whenfungalactivitypeakedinboththetreatedandnon-treated root samples. The number of total culturable bacteria was signi®cantly lowered by fenpropimorph at day 17 and stimulated at day 56, indicating a possible indirect effect of the fungicide on the culturable bacteria as a whole. Nevertheless, culturable Pseudomonas and actinomycetes were not affected. The succession of the two bacterial subgroups differed considerably, so, whereas Pseudomonas peaked in the early decomposition stages, actinomycetes were most abundant after 2 months, and on the very ®rst sampling day. Thus, the present ®ndingssuggestapossibleeffectoffenpropimorphonsoilfertility.ThediversityoftotalbacterialDNAmeasuredbydenaturinggradientgel electrophoresis DGGE) was unaffected by fenpropimorph treatment, there being a clear and highly reproducible succession in bacterial diversityduringdecompositionofthebarleyroots.Thediversityinthehotspotonthelastsamplingoccasionattwomonthswassimilartothe diversityofthebulksoil,thusindicatingareturntothelevelpriortoadditionoftheroots. q 2001ElsevierScienceLtd.Allrightsreserved. Keywords: Organic matter; Fenpropimorph; Actinomycetes; Pseudomonas; Fungi; DGCE 1. Introduction Fenpropimorph is a widely-used sterol biosynthesis- inhibiting leaf fungicide used for the control of Erysiphe graminis powdery mildew) and Puccinia recondita cereal rust).Recently,thesideeffectsoffenpropimorphonseveral organisms in soil have been studied, of which many are affected at recommended ®eld dose: it inhibits many other fungi Kuck and Scheinp¯ug, 1986; Steel et al., 1989; Loef¯er and Hayes, 1992) and, hence, is also likely to be activeagainstnon-targetsoilfungi.Ina®eldstudy,fenpro- pimorph signi®cantly inhibited the length of active saprotrophic fungal hyphae on ligni®ed wheat roots 50±114 days following application Bjùrnlund et al., 2000). On the other hand, various physiological parameters ofarbuscularmycorrhizalactivityinsoilwereunaffectedby fenpropimorph at recommended ®eld dose Kjùller and Rosendahl, 2000; Schweiger and Jakobsen, 1998). Some soil protozoa are also sensitive to fenpropimorph in recom- mended ®eld dose Ekelund et al., 1994; Ekelund, 1999). Using a similar microcosm set-up, we have previously shown that fenpropimorph in the formulation Corbel) is able to depress protozoa, thereby reducing predation pres- sure on bacteria Thirup et al., 2000). Whereas degradation of fenpropimorph has been seen Ebing et al., 1995; Stock- maier et al., 1996), bacterial degradation of fenpropimorph hasnotbeendescribed.Theliteratureonpesticideeffectsis enormous,butveryoftenstudiesareperformedinvitroand at unrealistic concentrations. Therefore, the growing body of literature of effects of fenpropimorph at recommended rate on soil microbial ecology improves our understanding Soil Biology & Biochemistry 33 2001) 1517±1524 0038-0717/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII:S0038-071701)00067-0 www.elsevier.com/locate/soilbio * Corresponding author. Address: Danish Veterinary Laboratory, Bu Èlowsvej 27, DK-1790 Copenhagen V, Denmark. Tel.: +45 35 30 01 00.Fax:+4535300120. E-mail address: kjo@svs.dk K. Johnsen).