Soybean [Glycine max (L.) Merr.] growers are concerned that soybean yield is restricted by limitations on biological N2 fixation and soil nitrogen (N) mineralization. However, a comprehensive study characterizing actual soybean N requirements across wide-ranging seed yield environments is nonexistent for modern soybean production systems. Using six site-years and eight soybean varieties, plants were sampled at six growth stages and partitioned into their respective plant parts and analyzed. For each kilogram increase in yield, total dry matter accumulation, harvest index, and total N uptake increased by 1.45 kg, 0.0034%, and 0.054 kg, respectively, but all varied by environment at any specific yield level, whereas N removal did not (0.055 kg N kg-1 grain). Nitrogen harvest index (NHI) increased (0.0019–0.004% kg-1 grain) with yield but varied by environment and yield level, resulting in indices between 73 and 90%. Peak uptake rates for N were 3.6 to 4.3 kg ha-1 d-1 between R4 and R5, depending on the yield level. After R5.5, 66 to 69% of vegetative N was remobilized to the seed, which accounted for 50.4% of seed N at the low yield level (3608 kg ha-1), but only 38.9% at the high yield level (5483 kg ha-1). Moreover, higher yields attained a greater portion of their total N uptake after R5.5 (40.1%) compared with the low yield level (29.7%). These results highlight greater remobilization efficiencies and late-season N uptake in conjunction with greater NHI to support higher yields per unit of N uptake in current production realities.
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The authors wish to thank John Gaska, Adam Roth, and Dimitri von Ruckert for their technical support and Dr. Jill Miller-Garvin and Dr. Jim Specht for their critical review. We would also like to thank the Wisconsin Soybean Marketing Board, DuPont Pioneer, and the University of Wisconsin-Madison, College of Agricultural and Life Sciences for funding this research.