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Background and Aims <p>The return-on-investment framework suggests that symbiotic nitrogen fixation (SNF) is carbon (C)-expensive and optimized for nitrogen (N) acquisition, implying its downregulation when N is abundant. However, many studies reveal paradoxical findings, with high SNF rates occurring under high N availability, often under conditions of drought, high light intensity, and elevated CO<sub>2</sub>.</p> Scope <p>Here we propose an alternative framework suggesting that C allocation to SNF is at least partly driven by plants transporting surplus C belowground, rather than being solely explained by N demand or availability. Under conditions like moderate drought, nutrient limitation, high light, or elevated CO<sub>2</sub>, plants may accumulate surplus C. For instance, moderate drought inhibits leaf growth but maintains photosynthesis, generating surplus C that could stimulate SNF through increased nodule biomass and SNF rates, even with low plant N demand.</p> Conclusions <p>Therefore, plant C availability may be a key factor regulating SNF. Adopting this surplus C perspective could improve ecological models, particularly for plant-microbial interactions under climate change scenarios. We recommend experimental validation involving isotopic tracing of C and N, and monitoring non-structural carbohydrate pools and SNF under conditions that induce C surplus. We suggest that plant surplus C provides a plausible, parsimonious explanation for many observations and should be considered when interpreting unexpected or paradoxical patterns in SNF.</p>

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Rethinking symbiotic nitrogen fixation: Could surplus carbon drive unexpected patterns of resource allocation?

  • 期刊论文

  • 出版模式：

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  • 发表日期：

    2026年2月20日

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  • Lindsay A. McCulloch,
  • Benton N. Taylor,
  • Nina Wurzburger,
  • Cindy E. Prescott

摘要

Background and Aims

The return-on-investment framework suggests that symbiotic nitrogen fixation (SNF) is carbon (C)-expensive and optimized for nitrogen (N) acquisition, implying its downregulation when N is abundant. However, many studies reveal paradoxical findings, with high SNF rates occurring under high N availability, often under conditions of drought, high light intensity, and elevated CO2.

Scope

Here we propose an alternative framework suggesting that C allocation to SNF is at least partly driven by plants transporting surplus C belowground, rather than being solely explained by N demand or availability. Under conditions like moderate drought, nutrient limitation, high light, or elevated CO2, plants may accumulate surplus C. For instance, moderate drought inhibits leaf growth but maintains photosynthesis, generating surplus C that could stimulate SNF through increased nodule biomass and SNF rates, even with low plant N demand.

Conclusions

Therefore, plant C availability may be a key factor regulating SNF. Adopting this surplus C perspective could improve ecological models, particularly for plant-microbial interactions under climate change scenarios. We recommend experimental validation involving isotopic tracing of C and N, and monitoring non-structural carbohydrate pools and SNF under conditions that induce C surplus. We suggest that plant surplus C provides a plausible, parsimonious explanation for many observations and should be considered when interpreting unexpected or paradoxical patterns in SNF.