Vertical distribution of endogonaceous mycorrhizal fungi associated with soybean, as affected by soil fumigation

Propagules of mycorrhizal fungi in a western Kentucky field highly productive for soybean were determined by quantitative bioassay ("most probable number") before and after fumigation with 67% methyl bromide-33% chloropicrin and before and after growing a crop of soybeans. Thirteen species of Endogonaceae were found. Gigaspora giganlea, Glomus macrocarpum, Glomus fecundisporum and Glomus maculosum were present primarily in the upper 15cm of soil, with few propagules at 30-45 cm. Of the species found at 30-45 cm, Gigaspora margarita and Glomus intraradices predominated. Soil fumigation eliminated most propagules in the upper 15 cm of soil but had little effect in the 30-45 cm layer. Effects of fumigation on soil NH⁺₄ and NO^-₃ concentrations were consistent with those on mycorrhizal fungi. After production of a crop of soybeans, the population of total propagules had completely recovered in fumigated soil to prc-fumigation numbers and was higher than the population in non-fumigated soil. G. margarita and Gl. intraradices were a higher proportion of the total community of propagules at the end of the season and may have had an advantage for recolonization because of their dominance at depths below the zone of effective fumigation. However, species which were less common deeper in soil also rccolonizcd the soil effectively. Soil Cl^- + Br^- concentration increased in fumigated soil due to degradation of fumigunts to an amount consistent with conventional fertilization with KCl.