Involvement of Phosphorus in Nitrogen Fixation by Subterranean Clover ( Trifolium subterraneum L.)

Effects of phosphorus supply on nodulation and nitrogen fixation in subterranean clover paralleled those on growth and occurred only after, or at the same time as, growth responses. However, correcting phosphorus deficiency increased nitrogen concentrations in tops as well as weight of tops. Effects of vesicular-arbuscular (VA) mycorrhizas on growth, nodulation and nitrogen fixation operated through effects on phosphorus nutrition of the host. Inoculation with a VA mycorrhizal fungus only stimulated nodulation and nitrogen fixation when insufficient phosphorus was applied for maximum growth of the non-mycorrhizal plant. Phosphorus concentrations in nodules greatly exceeded those in either tops or roots in both mycorrhizal and non-mycorrhizal plants and with phosphorus levels ranging from severely deficient to luxury supply for plant growth. Nitrogen applied after nitrogen fixation had commenced increased fresh weight of tops to a greater extent where phosphorus was non-limiting to growth. This positive interaction between nitrogen and phosphorus on the growth of nodulated nitrogen-fixing subterranean clover suggests that phosphorus deficiency does not limit growth in legumes by decreasing nitrogen fixation. It is concluded that increasing phosphorus supply increases nitrogen fixation in subterranean clover by stimulating host plant growth rather than by effects on either rhizobial growth and survival or on nodule formation and function.