Quantification of symbiotic nitrogen fixation by Elaeagnus angustifolia L. on salt-affected irrigated croplands using two 15N isotopic methods

Afforestation with fast growing N-fixing trees is an option for ecological restoration of highly-salinized irrigated croplands, but information about the N-fixing capability of trees on saline soils is sparse. The 15N-enrichment technique (15NET) and the A value (AV) method were used to quantify in lysimeters the proportion of atmospheric N2 (%Ndfa) fixed by Elaeagnus angustifolia L., with a reference to non-N-fixing Gleditsia triacanthos L. and Ulmus pumila L. Twenty kg N ha−1 of 5 atom %15N excess ammonium nitrate (35% N) was applied to 1-year-old trees in 2007 and 2-year-olds in 2008. Since this rate was insufficient for the older reference trees, 60 kg N ha−1 was applied in 2008. With 15NET, the %Ndfa of E. angustifolia in 2007 was 79% when referenced against U. pumila and 68% against G. triacanthos. With the AV method, the %Ndfa of 2-year-old E. angustifolia was 80 and 68% when referenced against U. pumila and G. triacanthos, respectively. Over 2 years, E. angustifolia fixed 17 kg N ha−1 when related to U. pumila and 14 kg N ha−1 with G. triacanthos (assumed density: 5,000 trees ha−1). N-fixing E. angustifolia has the potential to be self-sufficient in N when planted in the strongly saline soils.