RESPONSE OF SULFUR CYCLING MICROORGANISMS TO ARBUSCULAR MYCORRHIZAL FUNGI IN THE RHIZOSPHERE OF MAIZE

Fluctuations in the number of anaerobic sulphur mineralizers, autotrop hic sulphur oxidizers and sulphate-reducing microorganisms in pot cult ures of arbuscular mycorrhizal and non-mycorrhizal plants were examine d at 0, 15, 30, 45 and 60 days after planting using the most probable number enumeration method. Two arbuscular mycorrhizal (AM) fungi belon ging to different Glomus species were assayed. Populations of S-cyclin g microorganisms varied along the growth period for both AM fungi colo nized and uncolonized plants, with significant quantitative and qualit ative changes in the mycorrhizal compared with the non-mycorrhizal roo t zone, The occurrence of autotrophic sulphur oxidizers in pot culture s colonized with G. fasciculatum was significantly higher than in non- mycorrhizal ones throughout the plant growth period. The population si ze of this group was similar in non-mycorrhizal and G. mosseae coloniz ed soil, The population of anaerobic sulphur mineralizing microorganis ms was significantly decreased by mycorrhizal colonization. In contras t, pot cultures colonized by G. mosseae or G. fasciculatum had a highe r quantity of sulphate-reducing bacteria than the control. The fact th at the autotrophic S oxidizers and sulphate-reducing root-associated m icroorganisms were selectively stimulated by each one of the AM fungi assayed is a noticeable aspect. The maximum activity of the microbial processes regarding specific groups was at 15 days for the mineralizin g, 45 days for the oxidizing and 0 days for the reducing microorganism s. In the case of autotrophic S oxidizers, the maximum population was achieved by 30 days in G. fasciculatum colonized treatment. From the d ata on occurrence of physiological grouping of sulphur-cycling microor ganisms, we can deduce that a different ecological situation existed i n the rhizosphere developed by non-mycorrhizal plants compared with th ose colonized by each one of the mycorrhizal endophytes. (C) 1997 Else vier Science B.V.