Evaluation of abiotic factors and efficiency of Bradyrhizobium japonicum strains isolated from Argentine soils

Little is known about the native populations of B. japonicum in the soils o f the major soybean cultivation areas of Argentina. The growth response to six temperatures (20, 25, 30, 35, 40, 50 & 55 degrees C) and two soil moist ure contents (10 & 40% RH), were investigated in 8 biotypes of B. japonicum (USDA 138, G96, G39, A, B, C, D and E) isolated from two soils of the Prov ince of Buenos Aires. Two optimum growth temperatures were found: 28 degree s C to 30 degrees C for strains 138, G39 and G96; and 28 to 35 degrees C fo r strains: A, B, C, D, E. Strain populations increased in a sterile silt lo am soil at 40% RH, but rapidly declined at 10% RH. The effects of inoculati ng Asgrow 3127 soybean seeds with a water suspension of the 8 B. japonicum strains at seeding, on nodulation, N-2 fixation and physiological efficienc y were compared with a control under greenhouse conditions. Amount of N-2 f ixed by plants from Bradyrhizobium inoculated seeds were Significantly diff erent from the those fixed by plants from uninoculated seeds. Biotypes G39 and E fixed significantly (P = 0.05) more N-2 (2.48 and 2.51% N pl(-1) resp ectively) than any other strains. But higher N-2 fixation did not translate into more nodules pl(-1) or more dry matter. Physiological efficiency was maximum when seeds were inoculated with B, japonicum G96. N-2 Fixation and physiological efficiency of the strains were not significantly associated ( P = 0.05) under the conditions used.