DINITROGEN FIXATION, NITROGEN AND DRY-MATTER ACCUMULATION, AND NODULATION IN WHITE LUPINE

Timing of N uptake and N2 fixation may be important determinants of yield and seed N concentration in spring sown white lupine (lupinus albus L.). This study was initiated to determine N2 fixation, N and dry matter (DM) accumulation and distribution patterns, and nodulation as influenced by Bradyrhizobium sp. (lupinus) inoculation and N fertilizer. Inoculum strains WU425 and 96B15 were combined in a 1:1 cell ratio in inoculated plots and compared with noninoculated controls at 0, 56, 112, and 168 kg N ha-1 under irrigation on a loamy sand (Udorthentic Haploboroll) in 1989 and 1990. Control plots were maintained nodule-free. Bradyrhizobium strains persisted in this environment, and applied strains nodulated the tap root primarily. Increased total DM and N yield due to inoculation was first observed 42 days after emergence (DAE), and by 84 DAE, was more than twofold that of uninoculated 0-N controls in both years. Dinitrogen fixation, estimated by the difference method, was insignificant before flowering but totalled 157 kg N ha-1 and 196 kg N ha-1 by 84 DAE in 1989 and 1990, respectively. Inoculation or N fertilizer did not influence distribution of dry matter among plant parts. Previously observed enhancement of seed protein concentration due to inoculation was probably due to increased total plant N supply late in the season, not to more efficient mobilization of N to seed in inoculated plants. Supplemental N fertilizer suppressed nodulation and N2 fixation and did not improve early growth. These results indicate that spring-planted white lupine in effective symbiosis with bradyrhizobia are not normally N-limited at any time during growth under most conditions.