Endomycorrhizal fungal survival in continuous corn, soybean, and fallow

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 mar (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 cord 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.