Nitrogen Allocation By Soybean Cultivars Released Over 90 Years.

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. Our objective was to determine seasonal uptake of nitrogen (N) by soybean cultivars released over the last nine decades. We selected 25 cultivars from maturity group (MG) II and 26 cultivars from MG III that were released over the past 90 years and were representative of these decades. These cultivars were grown in nutrient-rich soil of west-central Indiana in 2011 and 2012. Each MG set was arranged in a randomized complete block design replicated three times. 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). As a result, 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.