Legume species identity and soil nitrogen supply determine symbiotic nitrogen-fixation responses to elevated atmospheric [CO₂]

• In nitrogen (N)-limited systems, the response of symbiotic N fixation to elevated atmospheric [CO₂] may be an important determinant of ecosystem responses to this global change. Experimental tests of the effects of elevated [CO₂] have not been consistent. Although rarely tested, differences among legume species and N supply may be important. • In a field free-air CO₂ enrichment (FACE) experiment, we determined, for four legume species, whether the effects of elevated atmospheric [CO₂] on symbiotic N fixation depended on soil N availability or species identity. Natural abundance and pool-dilution ¹⁵N methods were used to estimate N fixation. • Although N addition did, in general, decrease N fixation, contrary to theoretical predictions, elevated [CO ₂] did not universally increase N fixation. Rather, the effect of elevated [CO₂] on N fixation was positive, neutral or negative, depending on the species and N addition. • Our results suggest that legume species identity and N supply are critical factors in determining symbiotic N-fixation responses to increased atmospheric [CO₂].