RESPONSE OF NITROGEN-TRANSFORMING MICROORGANISMS TO ARBUSCULAR MYCORRHIZAL FUNGI

We studied fluctuations in the numbers of autotrophic ammonium oxidize rs, ammonifying microorganisms and denitrifying microorganisms in pot cultures of mycorrhizal and non-mycorrhizal maize. The populations wer e enumerated after 0, 15, 30, 45 and 60 days of plant growth. Two arbu scular mycorrhizal (AM) fungi belonging to different Glomus species we re investigated. Pot cultures with AM-infected maize had significant q uantitative and qualitative changes in the root-associated population of N-transforming bacteria compared with the non-mycorrhizal controls. The occurrence of autotrophic ammonium oxidizers in pot cultures of t he AM fungi Glomus mosseae and G. fasciculatum was significantly highe r than in non-mycorrhizal cultures throughout maize growth. The occurr ence of these bacteria was delayed by 15 days in non-mycorrhizal as op posed to Glomus-colonized soil. Ammonifying and denitrifying bacterial populations were significantly decreased in the pot cultures of AM pl ants compared with the control. The distribution patterns of the physi ological groups of bacteria tested were similar for both AM treatments but different from that of the non-mycorrhizal controls. Activity mea surements expressed on a per cell basis showed changes with respect to the form of N in the mycorrhizal soil. G. fasciculatum was more activ e than G. mosseae during the earlier stages of plant growth.