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

• In nitrogen (N)-limited systems, the response of symbiotic N fixation to elevated atmospheric [CO 2 ] may be an important determinant of ecosystem responses to this global change. Experimental tests of the effects of elevated [CO 2 ] have not been consistent. Although rarely tested, differences among legume species and N supply may be important. • In a field free-air CO 2 enrichment (FACE) experiment, we determined, for four legume species, whether the effects of elevated atmospheric [CO 2 ] on symbiotic N fixation depended on soil N availability or species identity. Natural abundance and pool-dilution 15 N 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 [CO 2 ] 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 [CO 2 ].