Ecotoxicology, 3, 4-10 (1994) Effects of the herbicide alachlor on soil microbial activities C. POZO, V. SALMERON, B. RODELAS, M.V. MARTINEZ- TOLEDO and J. GONZALEZ-LOPEZ* Group of Nitrogen Fixation, Department of Microbiology, Faculty of Pharmacy and Institute of Water Research, University of Granada, E-18071 Granada, Spain Received 23 March 1993; accepted 28 May 1993 A study was made of the effects of one selected acetanilide herbicide, alachlor, at concentrations of 2.0-10.0 kg ha-’ on bacterial populations, fungi, dinitrogen fixation bacteria, denitrifying bacteria, nitrifying bacteria, nitrogenase activity, acid and alkaline phosphatases, arylsulfatase and deshydrogenase. The presence of 2.0-10.0 kg ha-’ of alachlor in the soil increased the total number of bacteria and fungi. The population of denitrifying bacteria increased significantly at concentrations of 5.0-10.0 kg ha-‘. However, aerobic dinitrogen fixing bacteria and nitrogenase activity decreased at alachlor concentrations of 3.5-10.0 kg ha-‘. Acid and alkaline phosphatases, arylsulfatase and dehydrogenase activity decreased significantly initially at concentrations of 5.0- 10.0 kg ha-‘, but recovered to levels similar to those in the control. Nitrifying bacteria were not affected as a consequence of the addition of the herbicide to agricultural soil. Keywords: Alachlor; herbicides; pesticides; soil bacteria; enzymatic activities Introduction In recent years the influence of herbicides on microbial activities in soils has been studied by many investigators both in pure culture (Ferrer et al., 1986) and in mixed populations (Martinez-Toledo and Gonzalez-Lopez, 1989). However, it is not possible to reach a general conclusion regarding the effect of these substances on soil because a number of factors (for example, chemical composition of the herbicide, soil type and characteristic of the soil microbial community) influence the effects of these agrochemicals. Alachlor, 2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide, is a common herbicide in agriculture used to control broad-leaved annual and perennial weeds at concentrations in the range 2.0-5.0 kg ha-‘. The possibility that this herbicide may have an adverse effect on soil microbial activities could be of considerable importance, particularly as acetanilide herbicides are more persistent in soil than other herbicides such as phenoxy acid herbicides (Lin et al., 1988). The purpose of the present study, is to examine how the herbicide alachlor at concentrations approximating to the usual pest control levels affect soil microflora. The interactions established between the total number of bacteria, fungal populations, aerobic Nz-fixing bacteria, denitrifying bacteria, nitrifying bacteria, nitrogenase activity, *To whom correspondence should be addressed. 0963-9292 0 1994 Chapman & Hall