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Background and aims <p>Biological nitrogen fixation (BNF) supplies much of soybean (<i>Glycine max</i> (L.) Merr.) nitrogen (N) demand, but reported fixation rates vary widely, leading to uncertainty in the soybean N cycle. We quantified whole-plant soybean BNF and N allocation using long-term <sup>15</sup>N labeling to assess whether BNF can offset grain N removal in a high-yielding system, and to quantify root contributions to fixed N.</p> Methods <p>Field mesocosms in Iowa, USA received three legacy fertilizer N rates as 2 atom% <sup>15</sup>N-enriched urea for four prior seasons of maize (<i>Zea mays</i> L.) production. In 2024, N-fixing and non-N-fixing soybeans were grown to physiological maturity. Grain, aboveground biomass, and roots were analyzed to quantify BNF and to estimate whole-plant N budget.</p> Results <p>Soybean derived 81.8% ± 1% (SE) of total plant N from BNF. Legacy N rate had no effect on whole-plant N accumulation, BNF, or N partitioning among tissues (p &gt; 0.05). Roots contained ~2% of total fixed N, with most BNF-derived N allocated to grain. High grain N removal was largely offset by BNF (174.7 ± 8.9&#xa0;kg N ha<sup>−1</sup>), resulting in a near-neutral partial N balance (−5.7 ± 2&#xa0;kg N ha⁻<sup>1</sup>).</p> Conclusion <p>From this one-season mesocosm study, our finding of negligible fixed N in soybean roots indicates that neglecting root BNF does not necessarily bias conclusions about soybean N cycle. However, the combination of long-term isotope labeling and non-nodulating isolines yielded much less variable and higher BNF rates than previous studies, suggesting the possibility of underestimation for soybean BNF in historical studies.</p>

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Quantifying soybean biological nitrogen fixation using long-term isotope enrichment

  • 期刊论文

  • 出版模式：

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  • 发表日期：

    2026年7月14日

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  • Ophelia Tsai,
  • Steven J. Hall,
  • George L. Graef,
  • Luis G. Posadas,
  • James E. Specht,
  • William G. Crumpton,
  • Matthew J. Helmers,
  • Shawn P. Conley

摘要

Background and aims

Biological nitrogen fixation (BNF) supplies much of soybean (Glycine max (L.) Merr.) nitrogen (N) demand, but reported fixation rates vary widely, leading to uncertainty in the soybean N cycle. We quantified whole-plant soybean BNF and N allocation using long-term 15N labeling to assess whether BNF can offset grain N removal in a high-yielding system, and to quantify root contributions to fixed N.

Methods

Field mesocosms in Iowa, USA received three legacy fertilizer N rates as 2 atom% 15N-enriched urea for four prior seasons of maize (Zea mays L.) production. In 2024, N-fixing and non-N-fixing soybeans were grown to physiological maturity. Grain, aboveground biomass, and roots were analyzed to quantify BNF and to estimate whole-plant N budget.

Results

Soybean derived 81.8% ± 1% (SE) of total plant N from BNF. Legacy N rate had no effect on whole-plant N accumulation, BNF, or N partitioning among tissues (p > 0.05). Roots contained ~2% of total fixed N, with most BNF-derived N allocated to grain. High grain N removal was largely offset by BNF (174.7 ± 8.9 kg N ha−1), resulting in a near-neutral partial N balance (−5.7 ± 2 kg N ha⁻1).

Conclusion

From this one-season mesocosm study, our finding of negligible fixed N in soybean roots indicates that neglecting root BNF does not necessarily bias conclusions about soybean N cycle. However, the combination of long-term isotope labeling and non-nodulating isolines yielded much less variable and higher BNF rates than previous studies, suggesting the possibility of underestimation for soybean BNF in historical studies.