Extensive degradation of irrigated croplands, due to increasing soil salinity and depletion of soil nutrient stocks, is a major problem in Central Asia (CA), one of the largest irrigated areas in the world. To assess the potential for improving the productive capacity of degraded lands by afforestation, we examined N2 fixation of Elaeagnus angustifolia L. in mixed plantations with non-fixing Populus euphratica Oliv. and Ulmus pumila L. Fixation of N2 was quantified by the 15N natural abundance technique based on both foliar and whole-plant sampling during five consecutive growing seasons. Despite elevated root-zone soil salinity (6–10 dS m-1) and deficiency in plant-available P (4–15 mg kg-1), N2 fixation (%Ndfa) increased from an initial value of 20% to almost 100% over 5 years. Within each growing season, %Ndfa steadily increased and peaked in the fall. Annual N2 fixation, determined using foliar δ15N, initially averaged 0.02 Mg ha-1, peaked at 0.5 Mg ha-1 during the next 2 years and thereafter stabilized at 0.3 Mg ha-1. Estimates based on whole-plant δ15N were <10% lower than those based on foliar δ15N. The increase in plant-available soil N was significantly higher in E. angustifolia plots than in P. euphratica and U. pumila plots. Increases in the concentrations of organic C (19%), total N (21%) and plant-available P (74%) in the soil were significant irrespective of tree species. This improvement in soil fertility is further evidence that afforestation with mixed-species plantations can be a sustainable land use option for the degraded irrigated croplands in CA.