Wheat straw or poultry Pinus-sawdust litter were added to a forest acid soil and changes in the microbial community were studied during a 3-month soil incubation. Soil amendment did not alter the relationships between microbial groups. In all the samples most microbes were aerobic heterotrophic bacteria which predominated over fungal propagules and actinomycetes; fungal hyphae were relatively well developed, whereas the densities of cyanobacteria and algae were relatively low Among the N-cycle microbes, there was a relatively high number of ammonifiers, while both ammonium- and nitrite-oxidizers were scarce. Although soil amendment did not significantly alter the pattern of growth of most microbial groups, it drastically changed their numbers, alterations due to poultry addition being much more marked than those due to straw addition. Among the heterotrophic microbes, straw addition to soil increased fungal population but decreased bacteria and especially actinomycetes, while those that were able to carry-out ammonification were unaffected. Conversely, poultry manure addition favoured all the microbial groups, the increase in number following the order: bacteria > fungi > actinomycetes, the ammonium-producers being particularly stimulated. Autotrophic microbes, whether photo- or chemo-autotrophs, were decreased by straw and manure. The response of microbes indicated that the use of wheat straw could lead to a reduction in taxonomic and functional diversity in the microbial community and that, in general, poultry manure seems to be better for applying to soil. Copyright (C) 1996 Elsevier Science Ltd.