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We examined the interactive effects of elevated CO2, soil phosphorus (P) availability, and soil drought on nodulation, nitrogenase activity, and biomass allocation in Alnus hirsuta and Alnus maximowiczii. Potted seedlings were grown in either ambient or elevated CO2 (36 Pa and 72 Pa CO2), with different levels of P (7.7 and 0.77 mgP pot−1 week−1 for high-P and low-P, respectively) and water supply in a natural daylight phytotron. Measurements of nitrogenase activity by an acetylene reduction assay failed to reveal significant effects of the treatments in any species. In high-P, nodule biomass increased under elevated CO2 and decreased under drought. In low-P, nodule biomass decreased substantially compared to high-P, but the effect of elevated CO2 on nodule biomass was unclear. Soil drought increased the partitioning of biomass into nodules, especially in A. hirsuta. These results suggest that with high P availability, elevated CO2 could promote N2 fixation by increasing nodule biomass even under drought. On the other hand, if soil P is limiting, elevated CO2 may not enhance N2 fixation because of the suppression of growth.