Linseed was grown in field plots included in a long-term P fertilisation experiment (0, 15 or 30 kg P ha(-1) yr(-1) for 20 yr). Two months before sowing, half of each plot man applied with dazomet to prevent the formation of arbuscular mycorrhiza (AM). The biomass of different groups of micro-organisms was estimated 28, 51 and 72 d after sowing based on amounts of certain fatty acids extracted from the soil. Dazomet application strongly suppressed colonisation of the linseed roots by AM fungi throughout the experiment. In plots with no dazomet application, root colonisation by the AM fungi increased from harvests 1 to 3 as judged both from microscopical estimates and from quantitative analysis of the AM fungal indicative fatty acid 16.1 omega 5. These methods also revealed that AM formation was reduced in P-fertilised plots. The phospholipid fatty acid (PLFA) 16:1 omega 5 decreased in dazomet-treated soil, and it was assumed that the PLFA 16:1 omega 5 remaining in treated soil originated from bacteria. The biomass of the extraradical AM mycelium could then be estimated by multiplying the difference in PLFA 16:1 omega 5 between dazomet treated and nontreated soils by a conversion factor. This calculation indicated that the biomass of the extraradical mycelium of AM fungi was about 10 times as high as the biomass of intraradical mycelium and that the extraradical mycelium constituted the largest fraction of the soil microbial biomass. Dazomet application also decreased the biomass of saprophytic fungi in the soil as indicated by the amount of PLFA 18:2 omega 6,9, while analyses of bacteria-specific fatty acids indicated that the bacterial biomass in the soil was not affected by either dazomet or P application. (C) 1999 Elsevier Science Ltd. All rights reserved.