The Interplay of Light and Microbial Symbiosis in Shaping Plant Economic Spectrum Strategies

Legume-rhizobia symbiosis are fundamental drivers of nitrogen cycling and plant performance, yet their role in facilitating species strategies along the Plant Economic Spectrum (PES) remains insufficiently understood. We conducted a field experiment with four legume species subjected to light and shade treatments, with and without rhizobial inoculation, to assess plant survival, biomass accumulation, nodulation, nitrogen acquisition, and isotopic signatures, alongside microbial community diversity and shifts in vegetation composition. Results demonstrate that inoculation significantly enhanced survival, growth rates, nitrogen accumulation, and nodulation across species, particularly under light conditions, indicating that microbial symbiosis promotes acquisitive strategies within the PES framework. Contrary, shaded environments consistently favoured higher survival and root allocation but reduced growth, nodulation, and nitrogen fixation, reflecting more conservative resource-use strategies. Species-specific responses revealed differential PES positioning: Trifolium repens L. exhibited high acquisitive capacity under light, while Coronilla juncea L. showed poor survival and growth under both conditions, highlighting the interaction between phylogenetic identity and resource availability. Additionally, δ¹⁵N and %Ndfa values confirmed that inoculation increased nitrogen fixation efficiency, whereas microbial diversity analyses indicated strong shifts in soil bacterial communities associated with inoculated plants, suggesting feedback between symbiosis and soil microbiota. These findings support two main hypotheses: (i) rhizobial inoculation acts as a biotic driver promoting acquisitive strategies by enhancing resource acquisition and growth efficiency, and (ii) light availability serves as an abiotic axis that modulates species positions along the PES continuum. Together, our study provides novel evidence that both microbial interactions and resource availability jointly determine legume strategies within the PES.