Abstract
<title>Abstract</title> <p> <italic> <bold>Background and Aims</bold> </italic> Dryland leguminous shrubs meet part of their nitrogen (N) demand by symbiotic N₂ fixation, and the fixation–uptake balance sets their N economy and the new N returned to soil. In hyper-arid inland basins the operative stressor is secondary salinization, not spatially uniform aridity. We asked how soil salinity relates to the N-acquisition strategy of a woody halophytic legume and its coordination with leaf traits and growth. <italic> <bold>Methods</bold> </italic> <bold/> We surveyed Halimodendron halodendron in 24 plots spanning a salinity gradient (saturated-paste ECe 4.2–23.8 dS m⁻¹) in the middle–lower Tailan River, Tarim Basin, NW China, over two growing seasons. Atmospheric-N reliance (%Ndfa) was estimated from ¹⁵N natural abundance against non-fixing reference shrubs, alongside leaf economic, ion-regulation and water-use (δ¹³C) traits, soil chemistry and aboveground biomass. Structural equation modelling separated direct from trait-mediated salinity–biomass paths. <italic> <bold>Results</bold> </italic> <bold/> Soil water content and groundwater depth did not vary with salinity ( <italic>P</italic> ≥ 0.55), whereas available N declined and soil δ¹⁵N rose. Leaf δ¹⁵N fell from 5.9‰ to 1.7‰ and %Ndfa rose from ~ 22% to ~ 70% ( <italic>P</italic> < 0.001). Leaf N was maintained ( <italic>P</italic> = 0.61), whereas succulence, Na⁺/K⁺ and δ¹³C increased ( <italic>P</italic> < 0.001). Aboveground biomass declined via ion-regulation and stomatal-restriction traits, with no direct salinity–biomass path detected. <italic> <bold>Conclusion</bold> </italic> <bold/> Along this gradient, leaf N economy was defended while growth was not: atmospheric-N reliance rose as soil N availability fell, whereas salt-tolerance costs constrained biomass. Soil salinity is a distinct edaphic axis organizing legume N acquisition, ion regulation, water use and growth. </p>