Isolation and expression analysis of a plasma membrane Na+/H+ antiporter from Nitraria tangutorum Variation in Leaf Shapes of Nitraria Species and Effect on Leaf δ13C Isolation and Expression Analysis of a Vacuolar Membrane Na+/H+ Antiporter Gene NtNHX1 from Nitraria tangutorum Grid-Based Spatial Pattern of Nitraria tangutorumSand Mounds in Desert-Riverine Ecotones