Effect of rhizobia and soil nitrate on the establishment and functioning of the soybean symbiosis in the field

Abstract

The symbiosis of the root-nodules of Bragg soybean [Glycine max (L.) Merrill] and the relative dependence of the plants on symbiotic and soil sources of N were evaluated in an experiment conducted on a vertisol which was high in organic- and mineral-N, free of Rhizobium japonicum, and where poor nodulation was characteristic of inoculated, new sowings. Effective inoculant containing R. japonicum strain CB 1809 was sprayed into the seed bed at three rates of application (10-fold intervals). Increasing rates of inoculant led to greater numbers of rhizobia in the rhizosphere and in the soil, and to improved nodulation. Uninoculated plants did not nodulate. High soil NO-3 (30 ¦g N/g, top 30 cm) did not prevent prompt, abundant colonization of rhizospheres by the bacteria from the inoculant, but nodule initiation was delayed and nodule development was retarded until 42 days after sowing. There was an acceleration in nodule formation and development between 42 and 62 days which coincided with a depletion of NO-3 from the top 60 cm of the soil profile. Nodulated and unnodulated soybeans took up NO-3 at similar times and rates to a soil depth of 90 cm; only unnodulated plants utilized soil NO-3 below 90 cm. Vacuum-extracted stem (xylem) exudate was sampled from plants throughout growth and analysed for nitrogenous solutes. The proportion of ureide-N relative to total-solutes-N in xylem sap was used as an index of symbiotic N2-fixation. The initial increase in concentrations of ureides coincided with the period of accelerated nodule formation and development between 42 and 62 days. Thereafter, there was a progressive increase in ureide concentrations in nodulated plants, and the levels were related to rate of inoculation, extent of nodulation, and to the decline in concentrations of soil NO-3. Ureide concentrations in unnodulated plants remained low throughout. The quantities of NO-3-N and ß-NH2- N in xylem sap were not related to nodulation. The differences between treatments in terms of whole-plant N and grain N were less than predicted from the symbiotic parameters. This indicated that soybeans compensated for symbiotic deficiencies by more efficient exploitation of soil N and/or by more efficient redistribution of vegetative N into grain N, and that nodulation and soil NO-3 were interactive and complementary in meeting the N requirements of the crop.