Phosphorus Fertilization Reduces Vesicular‐Arbuscular Mycorrhizal Infection and Changes Nodule Occupancy of Field‐Grown Soybean1

Abstract

AbstractMycorrhizal infection of the roots of soybean [Glycine max (L.) Merr.] has been reported to stimulate both nodulation and N2 fixation, especially in soils low in available P. Less is known about yield responses of field‐grown soybean due to mycorrhizal infection, and little is known about the effects of mycorrhizal development on nodule occupancy by Bradyrhizobium japonicum strains. These field studies used two soils each of 2 yr to evaluate the changes in mycorrhizal infection, nodule occupancy, total nodulation, and seed yield when the level of mycorrhizal infection was reduced by adding 112 kg P ha −1 as triple superphosphate. Soils were chosen to represent a range of Bray PI extractable P (12–91 kg P ha −1), and belonged to Aquic Hapludolls, Typic Argiudolls, and Typic Haplaquolls. Mycorrhizal infection and nodulation were assessed, and the occupying bradyrhizobial strains identified at growth stages R1.0 in 1984 and R4.5 in 1985. Even though P fertilization significantly reduced mycorrhizal infection from 25.4 to 5.2 vesicles per 4‐cm root length, there was no effect on the number of nodules per plant or on the resulting seed yields. There was a significant change, however, in nodule occupancy; serogroup 123 averaged 72% in the nonfertilized plots and 37% in the plots fertilized with P. Because mycorrhizal reduction and P fertilization were confounded in the field studies, a greenhouse study was conducted where vesicular‐arbuscular mycorrhizal infection could be controlled by soil sterilization. Inoculation of soybean by Glomus mosseae in sterile soil did not increase the competitiveness of USDA 123 when challenged by USDA 110.