of propagules, and grow rapidly (Menge, 1985). Arbuscular mycorrhizal fungal species that adapt to a wide Survival of arbuscular mycorrhizal (AM) fungi in soil may be afrange of hosts and edaphic contingencies presumably fected by the presence or absence of crops and by the crop being grown. Field studies were conducted in central Iowa during three have a better chance of long-term survival. One means growing seasons with cropping to continuous corn (Zea mays L.) by which AM fungi survive is the production of spores (two cultivars), continuous soybean [Glycine max (L.) Merr.] (two although the length of survival of spores in field soils cultivars), or fallow in three soils to determine AM selection and under varying conditions is not well documented. Presurvival. The initial numbers of spores (all following soybean) in May sumably, when conditions are favorable, these spores 1996 averaged 0.9 g Ϫ1 soil in Clarion (well drained), 1.1 in Nicollet germinate, grow, and produce other fungal structures. (somewhat poorly drained), and 3.6 in Webster (poorly drained) soils. Many cornfields, especially in low-lying areas, exhib-In May 1998, the highest spore count average was 11.2 g Ϫ1 soil in Webster under corn and 3.0 spores g Ϫ1 under soybean. Nicollet soil ited stunting and purple coloration characteristic of seaveraged 6.8 spores g Ϫ1 for corn and 0.9 for soybean in May 1998, vere P deficiency in the fall following the central U.S. whereas Clarion soil had 6.3 for corn and 2.0 for soybean. The fallow floods of summer 1993 (Fixen et al., 1994). Plants treatments consistently had low spore counts, ranging from 0.7 to 1.0 seemed not to be absorbing sufficient P for fast early spores g Ϫ1 for all three soils. After 3 yr under the same cropping growth even though soil tests indicated adequate P. regime, spore numbers in soil were corn Ͼ soybean Ͼ fallow; no Farmers were advised to use 35 to 40 kg P ha Ϫ1 as starter significant differences were found between cultivars of the same crop. fertilizer in areas that had been flooded the previous Most probable number counts were correlated with spore counts and averaged 11% of spore counts, suggesting that only a portion of the year. Ellis (1998) conducted a survey to determine the spores were viable (or culturable in our determination). By the end status of AM fungal populations in areas that were of the study, Glomus albidum and G. etunicatum dominated under flooded in 1993 and reported a decline in AM fungal corn, whereas G. constrictum dominated under soybean.

Mycorrhizae are presumably important contributors to plant growth in most ecosystems. Our objective in this study was to evaluate the population diversity of arbuscular mycorrhizal (AM) fungi in Clarion (well drained) and Webster (poorly drained) soils of four Iowa soybean [Glycine max (L.) Merr.] fi elds. Th ese soils normally occur together on the landscape, with Clarion in the upland and Webster in lowland positions. Previous work consistently has shown higher AM fungal spore counts in Webster soils. Spore morphology was used for the characterization of fungal species. Identifi cation of certain species was confi rmed using polymerase chain reaction (PCR)-based rDNA fi ngerprinting . Glomus claroideum, G. etunicatum, G. mosseae, G. viscosum, and Paraglomus occultum-like spores were prevalent in both Clarion and Webster soils. Trap cultures led to detection of several addi- tional AM fungal species, including G. clarum, G. coronatum, G. fasciculatum, G. vesiforme, Acaulospora calossica, and Entrophospora infr equens. Th e richness of the Glomus species varied from eight species in Webster soil of a fi eld with highest available P (203 mg kg −1 soil Bray P1), where spores of G. mosseae constituted 90% of the spore population, to 12 species in Clarion soil, with lowest available P (14 mg kg −1 soil Bray P1). Th e variability of AM fungal density and distribution was also high among samples taken only 2 m apart. Although diverse within the genus, this study showed the dominance of Glomus species in fi eld soils used to produce two important commercial crops, corn (Zea mays L.) and soybean, in the midwestern United States.

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Survival of arbuscular mycorrhizal (AM) fungi in soil may be affected by the presence or absence of crops and by the crop being grown. Field studies were conducted in central Iowa during three growing seasons with cropping to continuous corn (Zea mays L.) (two cultivars), continuous soybean [Glycine max (L.) Merr.] (two cultivars), or fallow in three soils to determine AM selection and survival. The initial numbers of spores (all following soybean) in May 1996 averaged 0.9 g−1 soil in Clarion (well drained), 1.1 in Nicollet (somewhat poorly drained), and 3.6 in Webster (poorly drained) soils. In May 1998, the highest spore count average was 11.2 g−1 soil in Webster under corn and 3.0 spores g−1 under soybean. Nicollet soil averaged 6.8 spores g−1 for corn and 0.9 for soybean in May 1998, whereas Clarion soil had 6.3 for corn and 2.0 for soybean. The fallow treatments consistently had low spore counts, ranging from 0.7 to 1.0 spores g−1 for all three soils. After 3 yr under the same cropping regime, spore numbers in soil were corn > soybean > fallow; no significant differences were found between cultivars of the same crop. Most probable number counts were correlated with spore counts and averaged 11% of spore counts, suggesting that only a portion of the spores were viable (or culturable in our determination). By the end of the study, Glomus albidum and G. etunicatum dominated under corn, whereas G. constrictum dominated under soybean.