A quantitative method for analyzing nitrogen assimilation in soybeans

Seed yield in soybeans [Glycine max (L.) Merr.] strongly depends on nitrogen (N) assimilation (N_A). The N_A in nodulated soybeans is based on two N sources, soil and N fixation. Hence, the controlling N_A in soybean to increase seed yield is more difficult than in many other crops that depend on fertilizer. The objective of this study was to establish a quantitative relationship between N_A by plants and N applied via fertilizer (N_F) to increase soybean seed production. The N_A was modelled as a function of N_F with two parameters: amount of available soil N (N_S) and maximum N assimilation (N_Amax), both to be determined experimentally, and one equation to describe fixed N (N_FIX), would be maximum under nonfertilizer conditions (N_FIXmax). It is assumed that N_A is equal to N_Amax when N_F ≥ N_Amax - N_S and that, below this threshold, N_A decreases in proportion with N_FIX + N_S. To obtain variable datasets, different rates of N fertilizer were applied to nodulated and non-nodulated isolines under pot and field conditions from full bloom to beginning of maturity. Seed yields were found to be highly dependent on N_A regardless of nodulation type or genotype. In non-nodulated isolines, N_A - N_F was a good estimate of N_S. In nodulated soybeans, N_FIX could be modelled as a liner function of N_F. These results indicate that the relationship between N_A and N_F with N_S and N_FIX can be modelled in a simplified way. The relationships between N_FIX, N_F, and N_FIXmax indicate key traits for determining N_A for seed production in nodulated soybeans.