S oybean crops provide one of the world's most important sources of protein and oil. Historically, soybean yield improvements have occurred from biomass gains and increased partitioning to the seed, which all require large amounts of N (Balboa et al., 2018) supplied by BNF and/or the soil. In soybean, the contribution of N from BNF ranges from 0 to 98% depending on many factors, the most important being rhizobial activity. A past review on BNF documented an average contribution of 50 to 60%

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... vagiotti et al., 2008). Recent values recorded in Argentina (60%; Collino et al., 2015) fall within this range, but values of up to 80% have been noted in less fertile soils in Brazil (Alves et al., 2003). The main question motivating this review is whether BNF can supply sufficient N for high-yielding soybean systems (>7 t/ha) while maintaining a neutral partial N balance. Our data comprised 733 observations from 60 studies conducted from 1955 until 2017, including data on seed yield (adjusted to 13% moisture), BNF, and plant N uptake. A partial N balance was calculated as: Partial N balance = fixed N in aboveground biomass -N in harvested seeds. A negative partial N balance indicates that the amount of N exported in seeds is larger than N fixed, and thus a net "soil N depletion" occurs, which may affect the system N balance. The overall mean seed yield was 3.1 t/ha, and the maximum value was 8.3 t/ha (Figure 1) . Plant N uptake averaged 245 kg N/ha, with a maximum N uptake close to 560 kg N/ha. The slope of the middle regression line indicated that, on average, 81 kg plant N was required for every 1 t of soybean produced. However, Figure 1 shows a four-fold SUMMARY A review of 60 studies reporting on biological N fixation (BNF) in soybean was done to study the limits to which BNF can satisfy plant N demand. This review confirmed that BNF could satisfy plant N demand up to 200 kg N/ha. The N-gap (plant N uptake minus fixed N) widened rapidly if plant N demand exceeded 370 kg N/ha, which suggested the need for additional N under conditions of high yield potential. The partial N balance (fixed N minus N removed in seeds) was negative on average but approached neutral or positive values when BNF contributed at least 58% of plant N uptake. ABBREVIATIONS AND NOTES: N = nitrogen; BNF = biological nitrogen fixation.