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Changes in Soil Moisture, Temperature, and Salt in Rainfed Haloxylon ammodendron Forests of Different Ages across a Typical Desert–Oasis Ecotone

Soil water and salt movement during the freeze–thaw period facilitate soil and water conservation and agroecological environment maintenance in the desert–oases transition zone of the Hexi Corridor; however, our understanding of soil salinization and the shifting water, heat, and salt states in soil ecosystems of Haloxylon ammodendron forests at different ages is poor. We analyzed the soil moisture, temperature, and salinity characteristics of Haloxylon ammodendron forests of different ages in the Hexi Corridor of Northwest China and determined their coupling. Our results indicated that shallow (0–120 cm) soil temperatures significantly correlated with air temperatures. With increased forest age, the soil freezing period shortened and the permafrost layer shallowed. Changes in soil temperature lagged those in air temperature, and this lag time increased with forest age and soil depth. With increases in forest age and planting years, the water in the shallow soil layer gradually declined, and the surface aggregation of salt increased. In deep soils (120–200 cm), both soil moisture and salinity increased with the number of planting years. Accordingly, the clay layer and deep root system of Haloxylonammodendron greatly influenced the transport of soil water and salt; and temperature is a key driving force for their transport. Thus, water, temperature, and salt content dynamics were synergetic.