Phosphorus (P) is a key limiting nutrient for plant growth and development, but its bioavailability in soils is generally low. Inter-species interaction in mixed cultivation with atmospheric nitrogen (N2) fixing plants is considered an effective strategy for optimizing phosphorus resource acquisition and utilization efficiency. This study focused on the interaction of fast nodulating Robinia pseudoacacia and slow nodulating Alnus hirsuta seedling in a greenhouse experiment. The effect of different P supply levels (normal P at 1 mM and low P at 0.5 mu M) and planting modes (single and mixed planting) on growth, CO2 and H2O gas exchang, oxidative stress and anti-oxidative capacity were investigated. The results showed that at single planting, low P significantly reduced the biomass, photosynthesis and foliar total P (Ptot) contents of Robinia at 17.7 %, 33.4 %, and 10.2 %, respectively, while significantly enhancing nitrogenase activity in root nodules by 27.8 % and oxidative stress compensation in the leaves. The latter is manifested by a significant increase in the AsA/DHA and GSH/GSSG ratios in the leaves, with an increase of 83.3 % and 117.8 %, respectively. At the same time, the activities of GR and DHAR are enhanced by 143.2 % and 12.4 % respectively. At sufficient P supply, the biomass, nodule weight and N2-fixation capacity of Robinia were significantly inhibited at mixed cultivation with Alnus seedlings with reductions of 16.7 %, 22.0 %, and 61.9 %, respectively. Apparently, low P supply and mixed planting interacted by significantly amplifying the stress situation of Robinia. This interaction is indicated by a further decline of photosynthesis and N2-fixation capacity of Robinia, with reductions of 16.4 % and 48.5 %, respectively, reduced the antioxidant contents in leaves and roots, as well as as reduced GR activity (by 45.0 %). Thus, irrespective of P supply, Alnus seedlings have a negative impact on the performance of Robinia seedlings already before the N2fixing ability of Alnus seedlings has been established. These results indicated that geographic overlap is not a sufficient criterium to select tree species for mixed cultivation. Future greenhouse and field studies have to elucidate, if the negative impact of Alnus on Robinia is maintained beyond the seedling stage, when also Alnus becomes nodulated.