CAN FERTILIZATION OF SOIL SELECT LESS MUTUALISTIC MYCORRHIZAE

It has been noted previously that nutrient-stressed plants generally r elease more soluble carbohydrate in root exudates and consequently sup port more mycorrhizae than plants supplied with ample nutrients. Ferti lization may select strains of vesicular-arbuscular mycorrhizal (VAM) fungi that are inferior mutualists if the same characteristics that ma ke a VAM fungus successful in roots with a lowered carbohydrate conten t also reduce the benefits that the fungus provides a host plant. This two-phase study experimentally tests the hypothesis that fertilizing low-nutrient soil selects VAM fungi that are inferior mutualists. The first phase examines the effects of chemical fertilizers on the specie s composition of VAM fungal communities in long-term field plots. The second phase measures the effects of VAM fungal assemblages from ferti lized and unfertilized plots on big bluestem grass grown in a greenhou se. The field results indicate that 8 yr of fertilization altered the species composition of VAM fungal communities. Relative abundance of G i-gaspora gigantea, Gigaspora margarita, Scutellispora calospora, and Glomus occultum decreased while Glomus intraradix increased in respons e to fertilization. Results from the greenhouse experiment show that b ig bluestem colonized with VAM fungi from fertilized soil were smaller after 1 mo and produced fewer inflorescences at 3 mo than big blueste m colonized with VAM fungi from unfertilized soil. Fungal structures w ithin big bluestem roots suggest that VAM fungi from fertilized soil e xerted a higher net carbon cost on their host than VAM fungi from unfe rtilized soil. VAM fungi from fertilized soil produced fewer hyphae an d arbuscules (and consequently provided their host with less inorganic nutrients from the soil) and produced as many vesicles (and thus prov isioned their own storage structures at the same level) as fungi from unfertilized soil. These results support the hypothesis that fertiliza tion selects VAM fungi that are inferior mutualists.