VAM fungus spore populations and colonization of roots of maize and soybean under conventional and low-input sustainable agriculture

Spore populations of vesicular-arbuscular mycorrhizal (VAM) fungi and formation of mycorrhizae in maize (Zea mays L.) and soybean (Glycine max (L.) Merr.) were studied in three farming systems: a conventional maize-soybean rotation and two low-input systems. Spore populations were counted in soil samples obtained at planting and after harvest for two growing seasons. Maize and soybean root systems were sampled for mycorrhizae early in the growing season. Low-input plots tended to have higher populations of spores of VAM fungi than conventionally farmed plots. Further, the readily identifiable species Gigaspora gigantea (Nicol. & Gerd.) Gerdemann & Trappe, was more numerous in low-inputs plots (up to 30 spores 50 cm^-3 soil) than in conventional plots (0-0.3 spores 50 cm^-3 soil), suggesting farming system affected species distribution as well. Colonization of plants in the field did not always reflect VAM fungus spore populations at planting. Greenhouse bioassays showed 2.5-10 fold greater colonization of plants growing in soil from low-input than conventional systems. The results indicate that conventional farming systems yield lower levels of VAM fungi whereas low-input sustainable agriculture, with cover crops planted between cash crops, has greater populations of VAM fungi and potential to utilize the benefits of VA mycorrhizae.