Characterization of Nitrogen Fixation in US Soybean Systems

The demand for nitrogen (N) in modern soybean production can be exceptionally high. A soybean crop that yields 70 bu/A may take up 320 lb N/A, with half accumulated after full-pod stage (R4). The primary sources of N for soybean are biological N fixation (BNF) and inorganic soil N. Fifty to 60% of plant N may be derived from BNF, which declines with increasing mineral N in the soil.

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. Therefore, ureide concentration in the soybean stem represents an indirect measurement of how much N comes from fixation, and this approach is simpler and less expensive than the traditional ¹⁵N isotope method (a more direct measure of BNF). The objectives of this project are (1) to perform a regional characterization of soybean N fixation using the ureide technique, and (2) to explore the impact of exogenous soil N on seasonal BNF dynamics.

Thirty-three sites across the U.S. Corn Belt were established in 2016, and the experiment was replicated in 25 locations in 2017. Four soybean N treatments were imposed in a randomized complete block design: (1) zero N control, (2) 100 N lb/A applied at planting, (3) 100 N lb/A applied at V4 growth stage, and (4) 100 N lb/A applied at the R2 to R3 stage.

In 2017, grain yield ranged from 41 to 90 bu/A across sites and treatments, and yields were negatively correlated with protein concentration. Nitrogen fertilization had no effect on soybean yields, but significantly affected the seasonal BNF profiles. The maximum attainable N fixation consistently decreased when N fertilizer was applied at planting or the V4 growth stage, compared with unfertilized soybean. In contrast, the effect of N fertilization at the R2 to R3 stage was variable. The application of fertilizer at the R2 to R3 stage resulted in the highest reduction in BNF of the high N-fixing soybeans, but this effect was not observed for the moderate and low-fixing soybeans. The later application of N fertilizer resulted in a similar reduction as the other N treatment for the moderate-fixing soybeans, but had no effect on BNF of low-fixing soybeans. The changes in BNF throughout the entire growing season, combined with its variability across environments, reinforce the importance of properly characterizing BNF, and reveals opportunities for breeding efforts aimed to improve BNF.