The comparison of 15N natural abundance [?15N] between fixing and non-fixing reference species, is the most feasible method for nitrogen fixation studies in xeric shrubs. However, it assumes that both species use the same sources of soil N, the ?15N of such source is clearly different from zero, and the difference between the ?15N of both species is?%. On the other hand, the analysis of the difference between the ?15N of nodules and leaves [??15Nn-l] does not require a reference species. To evaluate the effect of water availability on nitrogen fixation, we analyzed the variation of ?15N and ??15Nn-l in watered and unwatered plants of Adesmia volckmanni Phil. [Fabaceae] and Mulinum spinosum [Cav.] Pers. [Apiaceae], formerly proposed as non-fixing reference species. However, we found that M.spinosum was not valid as reference species to study N fixation in Adesmia volckmann because its ?15N was closer to zero than that of the presumably fixing species, suggesting they use different soil N sources. In addition, water availability did not affect biological fixation after watering because both leaf ?15N and ??15Nn-l of A.volckmanni did not change either as soil dried up along the growing season nor in response to watering. However, the presence of nodules itself, the ?15N closer to zero than in a former dry year, and the ??15Nn-l [+1.23%] suggest that biological fixation could have occurred before watering, probably during early spring.