Seedlings of birch (Betula pendula Roth.) were grown for 12 weeks in small volumes of field soils heated in a microwave (MW) oven for different times and then either supplemented or not with inocula of ectomycorrhizal fungi and, in one case, with fresh agricultural soil. Growth of roots and shoots was assessed, and the incidence of different mycorrhizal types, distinguished by colour and morphology, was recorded. Bacterial and fungal populations were determined by dilution plating after MW treatment of soils. In four soils, shoot and root growth were markedly enhanced by MW treatments that raised the soil temperature to 60°C or more, irrespective of the exposure times (20-50 s) needed for this. Shoot dry weights were increased 1·9- to 2·9-fold by MW treatment of two soils from young (11-16 years) birch stands, compared with 8·0- and 11·4-fold for two soils from old birchwoods. But MW treatment of a colliery spoil reduced shoot growth 5-fold. The growth responses were not related to the size of seedlings in untreated soils. The responses coincided with substantial reductions in the total populations of bacteria and fungi. Hebeloma subsaponaceum Karsten developed similar numbers of mycorrhizas from mycelial inoculum added to untreated and MW treated soil. But Lactarius pubescent (Fr. ex Krombh.) Fr. developed significantly more mycorrhizas from inoculum added to MW-treated soil than to untreated soil. This difference between the fungi accords with their classification as, respectively, early stage and late stage in reported mycorrhizal successions on birch. Some mycorrhizal types developed on seedlings from resident inoculum in soil. Their incidence was reduced by different amounts following MW treatment, suggesting differences in heat-tolerance of their propagules. Hebeloma sacchariolens Quel., which forms sclerotium-like bodies and which probably developed from resident inoculum, was unaffected by MW treatment of soil. Some mycorrhizal fungi, notably Thelephora terrestris (Ehrh.) Fr., developed in place of others in soils treated for long times (80-180 s), and they probably developed from airborne spores.
Keywords: Key words: Microwave irradiation, ectomycorrhiza, birch growth, soil microorganisms, soil sterilization.