Characterization of Nitrogen Fixation in US Soybean Systems

Nitrogen (N) demands in modern soybean production can be exceptionally high. For example, one report stated that a 70 bu/A soybean crop takes up an aboveground total of around 320 lb N/A and approximately 50% of that is accumulated later in the season, after full-pod or R4 stage. The primary sources of N for soybean are biological N fixation (BNF) and inorganic soil N. One published report indicated that on average about 50 to 60% of required N comes from BNF. Although advances have been made in the understanding of BNF of soybean systems, improvements are needed to further the economic, environmental, and societal components of the sustainability of soybean production.

Nitrogen fixed by soybeans is assimilated and transported in the plant xylem in three forms: 1) ureides, 2) amino-N, and 3) nitrate. As nodulation activity increases, the ureide content in the xylem increases. Thus, ureide concentration in the soybean stem represents an indirect measurement of how much N comes from fixation. The so called “ureide approach” is simpler and less expensive than the traditional ¹⁵N isotope method, which is a more direct measure of BNF. The development of a field test using the ureide approach has the potential to be readily implemented an substantially improve N management of soybean systems. The objective of this project is to perform a regional characterization of soybean N fixation using both the conventional ¹⁵N and ureide techniques, and to calibrate the ureide method against the ¹⁵N technique.

Thirty-three sites across the U.S. Corn Belt were established in 2016 in cooperation with Winfield Solutions. Four soybean N treatments were imposed: 1) zero N control, 2) fertilizer N applied at planting, 3) N applied at V4 growth stage, and 4) N fertilizer applied later in the season at R2-R3 stage. At the time of this report, data from the N fixation tests were not available, but grain response to N application was. Grain yield across sites and treatments ranged from 38 to 97 bu/A. Although five of the 33 sites showed significant response to N application, no clear trend was observed. Data from the N fixation component of this study will soon be available due to this being the first year of this study.