The benefits of phosphorus fertilization of trees grown on salinized croplands in the lower reaches of Amu Darya, Uzbekistan

Afforestation of degraded croplands by planting N₂-fixing trees in arid regions is highly recognized. However, fixation of atmospheric nitrogen gas (N₂) by woody perennials is often limited on phosphorus (P) poor soils, while any factor limiting N nutrition inhibits tree growth. In a two-factorial field experiment, the effect of three P amendments was examined during 2006-2008 on N₂ fixation, biomass production, and foliage feed quality of actinorhizal Elaeagnus angustifolia L. and leguminous Robinia pseudoacacia L. With the ¹⁵N natural abundance method, N₂ fixation was quantified based on foliar and whole-tree sampling against three non-N₂-fixing reference species: Gleditsia triacanthos L., Populus euphratica Oliv., and Ulmus pumila L. The P applications, in March 2006 and April 2007 only, included (i) high-P (90 kg P ha^-1), (ii) low-P (45 kg P ha^-1), and (iii) 0-P. After 3 years, the average proportion of N derived from atmosphere (Ndfa, %) increased from 78 % with 0-P to 87 % with high P when confounded over both N₂-fixing species. With the used density of 5,714 trees ha^-1, the total amount of N₂ fixed (Ndfa, kg N ha^-1) with high-P increased from 64 kg N ha^-1 (year 1) to 807 kg N ha^-1 (year 3) in E. angustifolia and from 9 kg N ha^-1 (year 1) to 155 kg N ha^-1 (year 3) in R. pseudoacacia. Total above-ground biomass increases were too variable to be significant. Leaf N content and therewith also leaf crude protein content, which is an indicator for feed quality, increased significantly (24 %) with high-P when compared to 0-P for E. angustifolia. Overall findings indicated the suitability of the two N₂-fixing species for afforestating salt-affected croplands, low in soil P. With P-applications as low as 90 kg P ha^-1, the production potential of E. angustifolia and R. pseudoacacia, including the supply of protein-rich feed, could be increased on salt-affected croplands.