Response of sulphur cycling microorganisms to arbuscular mycorrhizal fungi in the rhizosphere of maize

Fluctuations in the number of anaerobic sulphur mineralizers, autotrophic sulphur oxidizers and sulphate-reducing microorganisms in pot cultures of arbuscular mycorrhizal and non-mycorrhizal plants were examined at 0, 15, 30, 45 and 60 days after planting using the most probable number enumeration method. Two arbuscular mycorrhizal (AM) fungi belonging to different Glomus species were assayed. Populations of S-cycling microorganisms varied along the growth period for both AM fungi colonized and uncolonized plants, with significant quantitative and qualitative changes in the mycorrhizal compared with the non-mycorrhizal root zone, The occurrence of autotrophic sulphur oxidizers in pot cultures colonized with G. fasciculatum was significantly higher than in non-mycorrhizal ones throughout the plant growth period. The population size of this group was similar in non-mycorrhizal and G. mosseae colonized soil, The population of anaerobic sulphur mineralizing microorganisms was significantly decreased by mycorrhizal colonization. In contrast, pot cultures colonized by G. mosseae or G. fasciculatum had a higher quantity of sulphate-reducing bacteria than the control. The fact that the autotrophic S oxidizers and sulphate-reducing root-associated microorganisms 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 mineralizing, 45 days for the oxidizing and 0 days for the reducing microorganisms. In the case of autotrophic S oxidizers, the maximum population was achieved by 30 days in G. fasciculatum colonized treatment. From the data on occurrence of physiological grouping of sulphur-cycling microorganisms, we can deduce that a different ecological situation existed in the rhizosphere developed by non-mycorrhizal plants compared with those colonized by each one of the mycorrhizal endophytes. (C) 1997 Elsevier Science B.V.