New Insights into Soybean Biological Nitrogen Fixation

Soybean biological N-2 fixation (BNF) relationships with fertilizer N and yield response have been comprehensively reviewed in the scientific literature. However, the study of the N-gap between N uptake and N supplied by N-2 fixation, and the partial N balance (fixed N in aboveground biomass - N seeds) needs further investigation. Therefore, the goals of this synthesis-analysis were to (i) quantify seed production per unit of fixed N under different amounts of N derived from the atmosphere (NDFA, %), (ii) study the N-gap and explore limitations of N-2 fixation (kg ha(-1)) for satisfying plant N demand, and (iii) calculate a partial N balance for soybean and determine its relationship with the N-2 fixation process. Data was gathered from 1955 through 2016 using studies reporting BNF, seed yield, and plant N uptake (n = 733 data points). The main outcomes of this review were (i) as NDFA increased, seed production per N-2 fixation decreased (from 0.033 to 0.017 Mg yield kg(-1) N from low, 28%, to high, 80%, NDFA); (ii) N-gap increased faster when NDFA values were above 80% and after plant N content was above 370 kg N ha(-1) suggesting that the crop needs additional N for coping yield potential; and (iii) when excluding roots, the partial N balance calculation revealed negative values across all NDFA levels. Future studies should consider a holistic approach to quantify the contribution of BNF in overall N cycling, including N contribution from roots, and to better understand the soil x plant x rhizobia interactions.