Nitrogen fixation by Elaeagnus angustifolia in the reclamation of degraded croplands of Central Asia

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 (610 dS m^-1) and deficiency in plant-available P (415 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.