Effect of Rhizobia and Soil Nitrate on the Establishment and Functioning of the Soybean Symbiosis in the Field

摘要

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