Response of mycorrhizal and P-fertilized soybeans to nodulation by Bradyrhizobium or ammonium nitrate

Includes bibliographical references (page 150).Management of N2-fixing bacteria or P-scavenging endomycorrhizae may lead to decreased fertilizer use on extensively cropped lands. To measure the effectiveness of these microsymbionts, soybean [Glycine max (L.) Merr. cv. Amsoy 71] plants were grown in a growth chamber in a soil [Josephine silty clay loam (mesic Typic Haploxerult)] low in plant-available N and P. Plants were inoculated with different Bradyrhizobium strains or received nutrient solutions of different N concentrations (0.0, 1.0, 2.0, 4.0 mM N) and P adequate for maximum plant growth under these conditions. Other plants were infected with a vesicular-arbuscular mycorrhizal (VAM) fungus and a Bradyrhizobium strain and received no N or P in the nutrient solution. The purpose of this study was to determine the growth response of soybean to N fertilization or nodulation by B. japonicum under conditions of high P availability or V AM-assisted P uptake. Nodulated non-VAM soybean plants had dry weights and development similar to that of the 4.0 mM N fertilizer treatment. Total N and Mn, leaf area, and leaf P of nodulated plants were higher than in the comparable N-fertilized plants in the absence of P stress. Soybeans infected with both the VAM fungus and Bradyrhizobium were similar in total dry weight, leaf area, and development to plants that received 1.0 or 2.0 mM N. They, however, contained more leaf N, more root Cu and Zn, and less Mn and P than the 2.0 mM N treatment. It is concluded that a number of host characteristics of nodulated plants are due to the altered functional aspects of the symbiosis and not N input alone. The presence of the VAM fungus can decrease nutrient stress in environments limited in P, Zn and Cu, elements essential in N2 fixation