Diversity and efficacy of arbuscular mycorrhizal (AM) fungi isolated from soils of soybean fields

Arbuscular mycorrhizal (AM) fungi, members of Glomeromycota, establish symbiotic associations with up to 90% of vascular plants and generally enhance plant diversity, plant growth, crop yield, and crop quality and contribute to the stability of the ecosystem. The net gains of the host plants from these associations are determined principally by characteristics of the plant and fungus entering the relationship. Thus, determination of the fungal diversity is fundamental to understanding the ecological effects of mycorrhizae. The first study in this project was a field survey that evaluated the population composition of AM fungal species in Clarion (a well drained fine-loamy, mixed, mesic Typic Hapludoll) and Webster (a poorly drained fine-loamy, mixed, mesic Typic Endoaquoll) soils of four Iowa soybean (Glycine max, L.) fields. Soil tests for available P ranged from 14 (low) to 40 (high) mg P kg"1 soil in Clarion soils, of fields 1 and 3, but those from fields 2 and 4 tested much higher (130 to 143 mg P kg"1 soil). Tests for Webster soils ranged from 38 to 69 in fields 1 and 3 and from 178 to 203 mg P kg"1 soil in fields 2 and 4. Average AM fungal colonization of field-collected soybean roots varied from 53 to 72% for most soil locations. Spore counts were <1 spore g"1 soil and significantly decreased in very high P soils. Spores from six species of Glomus and from the genera Acaulospora, Gigaspora, and Paraglomus were found in the original field soils. G. claroideum, G. etunicatum, G. mosseae, G. viscosum, and Paraglomus occultum-like spores were prevalent in both Clarion and Webster soils of all four fields. Trap cultures that involved BSR201 and Iowa2052 soybean plants grown in pots inoculated with Webster soil samples from fields 1 and 4 or BSR201, Iowa2052, Mandarin, and Peking soybean grown in pots inoculated with composite samples from Clarion or Webster soils of all four fields led to detection of several additional AM fungal species. The variability in the distribution of AM fungal species was larger among the fields than within the fields. The richness of AM fungal species varied from eight species in the Webster soil of field 2 with very high available P to twelve species in Webster soil of field 3 where the available P was not as high. Spores of G. mosseae constituted 90% of the spore population in the very high P Webster soil of field 4. Spore morphology-based identification of four Glomus species, G. claroideum, G. etunicatum, G. intraradices, and G. mosseae, plus the Gigaspora genus found in our soils were confirmed using polymerase chain reaction (PCR)-based rDNA fingerprinting protocols. Their presence in some roots was also verified either directly from the field samples or from pot cultures. Additional studies are needed to develop methods for the identification of AM fungal species directly in the roots of the host plants. In the second study, the selective ability for AM fungi of four soybean cultivars, BSR201, Iowa2052, Mandarin, and Peking, was assessed. The soybean cultivars were inoculated with composite soil samples collected from Clarion and Webster soils of the four soybean fields described in the preceding study. The soybean cultivars were harvested when they reached the reproductive stage R7 rather than all at the same time. A total of 12 morphotypes of AM fungal species were identified based on spores produced in the pots where the plants were grown. The distribution and richness of AM fungal species differed among the soybean cultivars. Pots of Iowa2052 soybean cultivar harbored all 12 AM fungal species. Spores of E. infrequens were found uniquely in Iowa2052 pots. Ten AM fungal species were identified in pots sown to BSR201 and Mandarin. Peking had only eight different types of AM fungi. The abundance of these species depended on the cultivar. For example, G. claroideum produced a high proportion of the spore population in pots of BSR201 (about 16 to 75%) and Iowa2052 (16 to 50%), medium in Mandarin (15 to 25%), and low in Peking (2 to 12%) when the inocula were derived from Webster soils. The rDNA fingerprinting using working species-specific primers confirmed the identity of spores and the presence in the roots of only four of twelve species. Subsequent work is needed to identify species that are still unknown or unconfirmed and to determine the distribution of additional species. Determination of the distribution and abundance of AM fungal species inside the roots is needed to better understand the AM fungal isolates actively colonizing roots. The efficacy of strains of G. claroideum, G. etunicatum, and G. mosseae was evaluated on the growth of BSR201, Iowa2052, and Peking soybean. The fungal strains differed either by the soil of origin, Clarion or Webster from fields 1 and 4, or by the cultivar used to propagate them, BSR201, Mandarin, or Peking. The fungal strains used in this study included five strains of G. claroideum, two strains of G. etunicatum, and one strain of G. mosseae. The inoculated 10-wk-old plants produced significantly higher shoot dry weights and seed