1) Quantify varietal differences for nodulation and BNF (biological Nitrogen fixation) using 15N isotope dilution methods. Fixed N will be measured in seed and residue, including the amount of N available to the next crop. 2) Assess how rate of N applied at seeding maximizes both BNF and yield. 3) Determine relationship between nodule number, BNF, and grain protein. 4) Investigate if nodule numbers can be modified by applying different amounts of Bradyrhizobium inoculant (specific for soybean). 5) Assess impact of low temperatures on nodulation, BNF, and productivity.