Rhizobiophage effects on nodulation,nitrogen fixation,and yield of field-grown soybeans (Glycine max L. Merr.)

Summary Previous laboratory and greenhouse studies have shown that phages significantly reduce soil populations of homologous rhizobia. Reductions in nodulation and N₂ fixation have also been observed. The purpose of the current study was to examine the effect of a phage specific ofBradyrhizobium japonicum USDA 117 on nodulation, nodule occupancy, N₂ fixation and soybean growth and yield under field conditions. The phage was inoculated in combination withB. japonicum USDA 117 and/orB. japonicum USDA 110 (resistant strain) into a rhizobia-free sandy loam soil and planted toGlycine max (L.) Merr. Williams. When the phage was applied to soil inoculated withB. japonicum USDA 117 alone, significant reductions in nodule weight and number, shoot weight, foliar N, nitrogenase activity, and seed index were observed. When, however, the soil also contained the non-homologous strain,B. japonicum USDA 110, no significant effects on any of these parameters were found. Nodule occupancy by competing strains ofB. japonicum USDA 110 and USDA 117 was also affected by the phage. In soil which did not contain the phage, 46% and 44% of the identified nodules were occupied by USDA 110 and 117, respectively. When the phage was present in the soil, nodule occupancy byB. japonicum USDA 117 was reduced to 23%, while occupancy byB. japonicum USDA 110 was increased to 71%. These results suggest that nodulation by selected strains of rhizobia can be restricted and nodulation by more effective, inoculated strains can be increased through the introduction of a homologous phage to soils.