Suppression of Mycorrhizal Fungus Spore Germination In Non-Sterile Soil: Relationship to Mycorrhizal Growth Response in Big Bluestem

Abstract

In six soils of varied phosphorus availability, spore germination of Glomus epigaeum was higher in non-sterile soil than in steamed soil and this was not altered by P amendment. In contrast, in all six soils without phosphorus amendment, spore germination of four other Glomus species was higher in steamed than in non-sterile soil. Amendment of phosphorus to steamed soil decreased percentage spore germination, whereas P amendment increased spore germination in non-sterile soil. To study the relationship between suppression of spore germination in non-sterile soil and subsequent plant growth response to mycorrhizal symbiosis and root colonization, plants were inoculated and grown in steamed soil, non-sterile soil, or steamed soil amended with non-sterile soil sievings. After 5 wk, seedlings were transplanted into additional soil of each treatment in all possible combinations. Exposure to the soil microflora and microfauna in the initial phase of the experiment was not deleterious to plant dry weight, but exposure after transplanting significantly reduced dry weight. However, significant reductions in root colonization by mycorrhizal fungi were imposed by both the initial and the post-transplant exposure to the soil microorganisms. Apparently, root colonization is a more sensitive measure of microbial suppression than dry weight, since relatively large changes in root colonization were necessary before dry weight was affected. Suppression of spore germination is a possible mechanism, though probably not the sole mechanism, explaining reduced plant growth and root colonization observed in non-sterile, low fertility soils.