90 Years of Soybean: More Nitrogen Required or Acquired?.

Advancements in soybean production has occurred through plant breeding and agronomic techniques over the past century. Soybean genetics and soil fertility are inherently related in production and research, but little research has documented changes in plant nutrition over this history. We have investigated nitrogen (N) contributions and limitations to soybean cultivars released over the past 90 years. As production potential of newer cultivars increases annually, N supply from soil and biological N fixation (BNF) may become a limiting factor. 

We applied a non-limiting supply of N fertilizer to soybean cultivars released from 1923 to 2008 in maturity group (MG) II and MG III in study 1 to determine (a) the yield potential and (b) if BNF met the N needs of newer, high yielding cultivars. Study 1 was conducted in 2010 and 2011 in four states. Fifty-nine MG II cultivars were used in Wisconsin and Minnesota, and 57 MG III cultivars were used in Illinois and Indiana. Nitrogen was supplied to the cultivars from two sources: soil N + BNF vs. soil N + fertilizer N. Fertilizer N totaled 560 kg N ha^-1in a split application at planting and V5. Biomass accumulation increased with cultivar modernization in both MGs, and N fertilization further increased biomass production in MG III cultivars. Yield increases of MG III cultivars from N fertilization were likely linked to increased N accumulation of vegetative biomass. Yield of MG II cultivars was not affected by N fertilization, but grain protein concentration increased in MG II cultivars with N fertilization. This suggested N supplied from BNF was insufficient to realize grain protein potential in MG II cultivars and yield potential in MG III cultivars released over the past nine decades.

Objectives of study 2 were to determine seasonal uptake and partitioning of N over the past 90 years. We selected 25 cultivars within each MG and grew them in nutrient-rich soil of west-central Indiana in 2011 and 2012. Cultivars were sampled at V4 (four expanding trifoliolates), R2 (full bloom), R4 (full pod), R6 (full seed), and R8 (physiological maturity) and were partitioned by leaves, stems, pods, and grain. Leaf N concentrations and accumulation in both MG II and MG III increased consistently across release years at every growth stage sampled (V4, R2, R4, and R6). Nitrogen accumulation at R4 in leaves, at R6 in leaves, and at R8 in grain increased at a faster rate after 1970. The proportion of N allocated to leaves at R6 notably increased across release years in MG II from 25 to 33% (1928 to 2011) and in MG III from 21 to 32% (1923 to 2011). Grain N accumulation has doubled over the past 90 years of soybean advancement. Leaf biomass, leaf N concentration, and leaf N accumulation increased across release years of soybean to prolong reproductive development, increase photosynthate production, and improve N remobilization efficiency to grain.