Environmental variables controlling biological nitrogen fixation in soybean

The effect of environmental factors on the proportion of the nitrogen (N) derived from the atmosphere (Ndfa) in soybean [ Glycine max (L.) Merril] have been traditionally approached individually given their intrinsic complexity. Alternatively, a more in-depth investigation of such complex interactions can be pursued by delimiting environments where Ndfa is uniform. Thus, the aim of this study was to define environments on 24 sites by extensively characterizing weather, plant, and Ndfa-related traits to find discriminant variables defining three Ndfa classes (low, medium, and high). The Ndfa was determined at the beginning of the seed filling (R5 growth stage) using the 15 N natural abundance method. Twenty environmental variables were utilized to categorize Ndfa in low (