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Dinitrogen Fixation, Nitrogen and Dry Matter Accumulation, and Nodulation in White Lupine

Dep. of Agronomy and Plant Genetics

Carroll P. Vance

USDA-ARS

Peter H. Graham

Dep. of Soil Science, Univ. of Minnesota, 411 Borlaug Hall, St. Paul, MN 55108

^* Corresponding author.

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 O-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.

Received for publication September 16, 1991.