Microbial community composition and enzyme activities in a sandy loam soil after fumigation with methyl bromide or alternative biocides Susanne Klose a, * , Veronica Acosta-Martı ´nez b , Husein A. Ajwa a a Department of Plant Sciences, University of California at Davis, 1636 East Alisal Street, Salinas, CA 93905, USA b USDA-ARS, Cropping Systems Research Laboratory, 3810 4th Street Lubbock, TX 79415, USA Received 17 February 2005; received in revised form 8 July 2005; accepted 28 September 2005 Available online 8 November 2005 Abstract A sandy loam soil was fumigated in microcosms for 24 h with methyl bromide and chloropicrin (MeBrCCP), propargyl bromide (PrBr), combinations of 1,3-dichloropropene and CP (InLine), iodomethane and CP (Midas), an emulsifiable concentrate of CP (CP-EC), or methyl isothiocyanate (MITC). The effects of these pesticides on fatty acid methyl ester (FAME) profiles and selected enzymatic activities were evaluated in fumigated soils and a nonfumigated control at 1, 3, 7, 14, 21, 28, and 90 days post-fumigation. Bacterial (a15:0, i15:0, i16:0, cy17:0, a17:0 and i17:0) and fungal (18:2u6, 18:3u6, 18:1u9) FAMEs were initially (1 day post-fumigation) reduced by fumigation with CP-EC, InLine, and Midas. Microbial communities of soils fumigated with MeBrCCP, MITC, and PrBr resembled those of the control soil. At 14–28 days post- fumigation, FAME profiles were changed in all fumigated soils relative to the control, with the exception of soils treated with MITC. At 90 days post-fumigation, FAME profiles suggested that actinomycetes (10 Me 16:0, 10 Me 17:0, 10 Me 18:0) and Gram-positive bacteria may recover preferentially after fumigation with most of the pesticides studied. Among the fumigants tested, InLine, Midas, and CP-EC had a higher potential to alter the microbial community structure in the longer term than MeBrCCP, PrBr and MITC, with MITC having the least effect. Soil enzyme activities in fumigated microcosms were significantly (P%0.037) different from the nonfumigated soil, with the exception of b-glucosidase in soils treated with PrBr and MITC, and dehydrogenase in MeBrCCP-fumigated soils. Over the 90-day study, soil fumigation (average of all fumigants and sampling dates) reduced the activities of arylsulfatase (62%), dehydrogenase (35%), acid phosphatase (22%), and b-glucosidase (6%), suggesting that S mineralization in soils and the total oxidative potential of microorganisms were more affected by fumigation than P and C mineralization. This study also indicates that soil fumigation with MeBrCCP alternative biocides has the potential to alter microbial communities and important key reactions involved in nutrient transformation. q 2006 Elsevier Ltd. All rights reserved. Keywords: Methyl bromide alternatives; Soil fumigation; FAME profiles; Enzyme activities; Carbon dynamics and nutrient availability 1. Introduction Preplant soil fumigation with methyl bromide (CH 3 Br, MeBr) and chloropicrin (CCl 3 NO 2 , CP) has been used widely around the world to control insects, nematodes, weeds, and pathogens such as Phytophthora cactorum, P. fragariae, Verticillium dahliae and Colletotrichum acutatum in many vegetable, fruit, nut, ornamental and nursery crops (Ajwa et al., 2003b; Tanaka et al., 2003). A great portion of the MeBr can potentially escape into the atmosphere during or shortly after the application, contributing to the depletion of the strato- spheric ozone layer (Yung et al., 1980; Prather et al., 1984). In accordance with the Montreal protocol, the import and manufacture of MeBr in the USA and other developed countries will be banned by 2005, after stepwise reductions in 1999, 2001, and 2003 (USEPA, 1993; UNEP, 1997). The loss of MeBr will greatly affect agricultural, silvicultural, and horticultural production unless safe and efficacious alternatives are found. Commercially available alternatives to MeBr are CP, 1,3-dichloropropene (C 3 H 4 Cl 2 , 1,3-D), InLine (1,3-D plus CP), and methyl isothiocyanate (MITC) generators such as sodium methyldithiocarbamate (CH 3 NHCSS Na, trade name metam sodium) and potassium methyldithiocarbamate (CH 3- NHCSS K, trade name metam potassium) (Ajwa et al., 2003b). Currently, experimental chemical alternatives to MeBr are iodomethane (CH 3 I, trade name Midas) and propargyl bromide (C 3 H 3 Br, PrBr) (Ajwa et al., 2001; 2003b). Previous studies provide information about the biological degradation of various alternative fumigants in soil (Miller et al., 1997; Gan et al., 1999; 2000; Di Primo et al., 2003) and Soil Biology & Biochemistry 38 (2006) 1243–1254 www.elsevier.com/locate/soilbio 0038-0717/$ - see front matter q 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.soilbio.2005.09.025 * Corresponding author. Tel.: C1 831 755 2805; fax: C1 831 755 2898. E-mail address: sklose@ucdavis.edu (S. Klose).