Response of free-living soil protozoa and microorganisms to elevated atmospheric CO 2 and presence of mycorrhiza

Abstract

Possible interactions between mycorrhiza, atmospheric CO 2, free-living soil microorganisms and protozoa were investigated in pot experimental systems. Pea plants ( Pisum sativum L. cv. Solara) were grown under ambient (360 μl l −1) or elevated (700 μl l −1) atmospheric CO 2 concentration with or without the presence of the arbuscular mycorrhizal (AM) fungus Glomus caledonium. It was hypothesised that (1) the populations of free-living soil protozoa would increase as a response to elevated CO 2, (2) the effect of elevated CO 2 on protozoa would be moderated by the presence of mycorrhiza and (3) the presence of arbuscular mycorrhiza would affect soil protozoan numbers regardless of atmospheric CO 2. After 3 weeks growth there was no difference in bacterial numbers (direct counts) in soil, but the number of free-living bacterial-feeding protozoa was significantly higher under elevated CO 2 and was significantly reduced in the mycorrhizal treatments. These effects on protozoa disappeared after 5 and 9 weeks. Neither mycorrhiza nor CO 2 concentration had any substantial effect on the microbial community structure as evaluated by phospholipid fatty acid analysis. The increased protozoan populations under elevated CO 2 suggest increased bacterial production, whereas the lower populations in response to presence of mycorrhiza suggest a depressing effect on bacterial production by AM colonisation.