Rhizobium japonicumNodular Occupancy, Nitrogen Accumulation, and Yield for Determinate Soybean Under Conservation and Conventional Tillage1

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Abstract
Nitrogen fixation and accumulation are important facets of soybean (Glycine maxL.) production. Three determinate soybean cultivars were grown on a Norfolk loamy sand (Typic Paleudult) with either conservation or conventional tillage. Plots were split for inoculation with strain 3I1b 110 of Rhizobium japonicum. Dinitrogen fixation was estimated by the difference between total shoot N for nodulating and nonnodulating ‘Lee’ isolines, and net N returned to the soil was estimated by the difference in dinitrogen‐fixed N and seed N. Drought was significant the first year; only 19.5 cm of rain fell during the podfill period. Rainfall and soil water were adequate the second year. Tillage, cultivar, and inoculation did not consistently affect the percentage of nodules formed by particular rhizobial strains, individually, but specific treatment combinations significantly affected nodular occupancy by certain strains. Inoculated ‘Coker 338’ had the highest shoot total N accumulation in both years; however, the difference between years overshadowed tillage, cultivar, or inoculation effects. For the 2‐y period, percentages of N supplied by N2fixation were estimated to be 58 to 67% and 49 to 65% under conservation and conventional tillage, respectively. Yields generally did not differ significantly for tillage or inoculation; however, Coker 338 soybean was lowest in seed yield even though it had the highest total N accumulation in the shoots. Estimates of net N returned to the soil varied more between years than between tillage treatments; values ranged from 14 to 40 kg ha−1in 1980 and 57 to 123 kg ha−1in 1981.