Suppression of vesicular–arbuscular mycorrhizal fungus spore germination by nonsterile soil

When the effect of nonsterile soil on vesicular–arbuscular mycorrhizal fungus spore germination was examined, significantly fewer Glomus etunicatum (22 vs. 63 and 64%) or Glomus mosseae (23 vs. 80 and 79%) spores germinated in nonsterile soil than in autoclaved or pasteurized soil, respectively. In some cases, addition of nonsterile soil sievings to autoclaved or pasteurized soil also reduced germination, as compared with germination in unamended pasteurized or autoclaved soil. Germination was reduced by as much as 45% in autoclaved or pasteurized soil if spores were surface sterilized. However, surface sterilization of G. etunicatum spores did not affect dry weight of inoculated big bluestem plants, but mycorrhizal root colonization was reduced when spore-associated microbes were removed from spores by surface sterilization. The detrimental effect of soil microbes on spore germination and mycorrhizal growth response may reflect microbial competition for nutrients since germination of G. etunicatum and G. mosseae was reduced when pasteurized soil was amended with 15, 30, or 60 ppm phosphorus and 60 ppm phosphorus, respectively. An optimum range of available phosphorus may exist, above or below which germination is suppressed.