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 CO₂.</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 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.</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>